Thermoelectric device

ABSTRACT

The present invention provides a thermoelectric device comprising a first electrode, a second electrode, and conducting composition capable of conducting ions, wherein the first and second electrodes are ionically coupled via said conducting composition such that an applied temperature difference over said conducting composition or an applied voltage over said electrodes facilitate transport of ions to and/or from said electrodes via said conducting composition, and wherein said conducting composition capable of conducting ions comprises a polymeric electrolyte. There is also provided a method for generating electric current and a method for generating a temperature difference.

FIELD OF THE INVENTION

The present invention is directed to thermoelectric devices, such asthermoelectric generators (TEG) for converting a temperature differenceinto electricity, and thermoelectric coolers for pumping heat withelectrical power.

BACKGROUND TO THE INVENTION

A conventional thermoelectric device is a semiconductor device thatconverts a temperature difference into electricity or vice versa. Themost common thermoelectric device is the thermoelectric generator (TEG),which converts a temperature difference into electricity, and which iscomposed of semiconductor legs, p-type and n-type legs connected inseries electrically, and in parallel thermally. The thermoelectricmaterial in TEGs is generally characterized with three fundamentalproperties: a high electrical conductivity (σ), a large Seebeckcoefficient, α and a low thermal conductivity (λ). Those properties aregathered in the so-called thermoelectric figure-of-merit (ZT), whereZT=σα²T/λ, and Z is a measure of a material's thermoelectric properties,T is the absolute temperature. The Seebeck coefficient is a measure ofthe magnitude of an induced thermoelectric voltage in response to atemperature difference across that material, which may also be calledthe thermopower or thermoelectric power of a material. In order toachieve a thermoelectric material with high efficiency, the materialshould preferably have both high thermodiffusion and low thermalconductivity at the same time. However, it is difficult to findmaterials with these characteristics. The importance of the materialproperties, its ZT values, is due to the fact that it is directlyconnected with the efficiency of the thermoelectric devices. High ZTmaterials give efficient devices.

The thermoelectric cooler, which pumps heat from one side to the otherside to create a temperature difference thanks to an electrical power,is also a thermoelectric devices, called Peltier cooler.

Currently, thermoelectric devices are mostly used as for example inPeltier coolers, temperature sensors However, they are regarded to havegreat potential for electricity production from waste heat and naturalheat source (geothermal, solar) in the future, but in order to be usedas large-area heat exchangers or in combination with large area solarcells, there is a need to develop TEGs which are suitable for largeareas and suitable for low-temperature applications (below 200° C.).

Several families of materials have been considered as thermoelectricmaterials: semi-metals, metal-oxide, and inorganic semiconductormaterials. Up to now, the best thermoelectric materials for use attemperatures up to 200° C. are heavy metal alloys composed of lownatural abundance, such as for example Bi₂Te₃. This material is usedsince it has both high Seebeck effect and low thermal conductivity. Thethermoelectric figure-of-merit is close to 1. However, it would be veryexpensive to create a large area heat exchanger from this material. Inaddition, the toxicity is a disadvantage.

There are only few studies on the use of organic materials forthermoelectric applications since organic materials have been regardedto have too low thermoelectric figure-of-merit to be interesting for usein thermoelectric devices. Recently, the thermoelectric properties of amodified conductive polymer has been investigated, see Optimization ofthe thermoelectric figure of merit in the conducting polymerpoly(3,4-ethylenedioxythiophene) Bubnova et al, Nature Materials vol.10, June 2011. Here, the p-type leg is made of PEDOT-Tos(PEDOT=Poly(3,4-ethylenedioxythiophene), Tos=tosylate) treated withtetrakis(dimethylamino)ethylene (TDAE) and the n-type leg is obtainedfrom an organic salt TTF-TCNQ. Both legs are electrically connected to atop Au electrode. The thermoelectric figure-of-merit for this conductingpolymer was found to be 0.25.

However, there is still a great demand for TEGs for low temperatureapplications (below 200° C.) having high thermoelectric figure of meritand which can be produced at low cost and still be environmentallyfriendly.

SUMMARY OF THE INVENTION

One object of the invention is to overcome or at least alleviate one ormore of the above mentioned drawbacks.

This and other objects are met by the subject matters provided in theindependent claims. Preferred embodiments of the invention are presentedin the dependent claims.

As a first aspect of the invention, there is provided a thermoelectricdevice comprising a first electrode, a second electrode, and aconducting composition capable of conducting ions,

wherein the first and second electrodes are ionically coupled via saidconducting composition such that an applied temperature difference oversaid conducting composition or an applied voltage over said electrodesfacilitate transport of ions to and/or from said electrodes via saidconducting composition, and

wherein said conducting composition capable of conducting ions comprisesa polymeric electrolyte.

According to an embodiment there is provided a thermoelectric devicecomprising

a first electrode comprising a first conductive polymer compositioncapable of being reduced and/or oxidized, and

a second electrode comprising a second conductive polymer compositioncapable of being reduced and/or oxidized;

at least one conducting composition:

wherein the first and second electrodes are ionically coupled via theconducting composition such that an applied temperature over theelectrolyte composition or an applied voltage over the electrodesfacilitate transport of ions to and/or from the electrodes via theconducting composition, thereby driving a reduction-oxidation (redox)reaction comprising the conductive polymers at the electrodes.

According to an embodiment, the conducting composition is furthercapable of conducting electrons.

The thermoelectric device as defined above is based on a new mainprinciple. While prior art devices are based on electron transportwithin the thermoelectric device, the present invention utilizes iontransport or ion transport together with electron transport. Advantagesof the present invention are, for instance, high efficiency, lowmanufacturing costs and printability. Since all the functional materialsfor fabricating the thermoelectric device can be solution based, it isfeasible to implement the thermoelectric devices on a flexible substrateby printing or other liquid deposition techniques.

In other words, while prior art devices relies on electron transport inorder to obtain a thermoelectric effect (Seebeck effect), the presentinvention utilizes ions or ions and electrons as charge carriers. In thepresent invention, the obtained ionic thermoelectric effect is a directconsequence of the transport, e.g. thermodiffusion, of ions (Soreteffect), or ions and electrons, and the specific components of theconstructed devices.

Areas wherein the present invention may be used are, for instance, inwaste heat management in electricity production (power plants), processindustries, transportations or other places where a lot of heat isnormally lost to the surroundings. The present invention may also beused in renewable power sources, such as geothermal or solar heatsources or advantageously in combination with solar cells.

The transported ions may be transported via the conducting compositionbetween the electrodes or from a storage, such as from a connector, tothe electrodes.

The term “driving a reduction-oxidation (redox) reaction” refers to thetransport of ions triggering reduction of a conducting polymer at oneelectrode and oxidation of a conducting polymer at the other electrode,respectively.

Thermoelectricity allows for reversible interplay between heat flow(temperature gradient) and charge flow (electricity current). Athermoelectric effect may be obtained in various ways. A thermoelectriceffect wherein a heat flow transport charge carriers, thus producing avoltage, is an electric power source (the Seebeck effect). A reverseeffect wherein an electrical current is used to generate a heat flow(Peltier effect) and create a temperature gradient. A third kind ofthermoelectric effect is the so called Thomson effect wherein atemperature gradient together with an electrical current cause heat tobe generated and absorbed, respectively.

In other words, the present invention relates to a thermoelectric devicethat utilizes a thermoelectric effect for producing an electric currentdue to a temperature gradient, and to a thermoelectric device forconverting an electric current into a temperature gradient.

Transport of the ionic current is achieved by the electrolytecomposition and the electrochemical reaction, i.e. the redox reactionsat the electrodes, produces electrons that may be used in an externalelectric circuit.

The ions may be transported by means of thermodiffusion.

Thermodiffusion, also called thermophoresis or Soret effect, is aphenomenon observed when a mixture of two or more types of mobileparticles subjected to a temperature gradient and the different types ofparticles respond to it differently. The term Soret effect normallyimplies thermodiffusion in liquids.

As used herein, ionic contact between a first and a second part of thedevice or “ionic coupled” means that the parts are arranged so that itis possible for ions to move from the first part to the second partand/or vice versa. This can be achieved by using direct physical contact(common interface) or by creating an ion transport path between thefirst and second part using an ion transporting material in physicalcontact with the first and second part. Additionally, the term directionic contact used herein means direct physical contact allowing forions to be transported between the two parts.

Devices of the present disclosure may also be coupled in series to formthermoelectric assemblies.

The electrolyte composition may be comprised in or form a “leg” of thedevice. Thus, the device may comprise one or several legs for transportof the ions. A leg may be spatially separated from another leg so as toprevent transport of ions of opposite charge in the same leg.

In embodiments of the first aspect, the conducting composition iscomprised in a single leg between the first and second electrodes.

In such an embodiment, the applied temperature difference may be betweenthe two electrodes, thus driving ions and possibly electrons from oneelectrode to the other via the conducting composition. All transportedions may then be of the same sign. If both electrons and ions aretransported, then anions and electrons may be transported in the singleleg.

Consequently, when a temperature gradient or a temperature difference isimposed between the two electrodes, the ions may thermo-diffuse throughthe leg comprising an electrolyte composition towards the cold side andgenerate a measurable thermo-voltage.

The substrate carrier, i.e. the single leg, of a single leg device, ispreferably a good thermal conductor and a good electrical insulator.Thus, a thick metal film coated with a thin polymer film may beadvantageous to use since it may also be flexible.

A single leg device, such as a single leg device without any ionreservoirs, may be sandwiched between two layers of a material, orcomposition of materials, of high thermal conductivity and lowelectrical conductivity. This may be advantageous for verticalstructures since it may provide more power.

The term “conducting composition” refers to a composition with thecapacity of conducting both ions and electrons. Thus in embodiments, theconducting composition is capable of conducting both electrons and ions.

As an example, the at least one conducting composition may comprise atleast one conducting polymer and at least one polyelectrolyte, or atleast one conjugated polyelectrolyte.

If more than one conducting composition is used, then the compositionsused should preferably not have opposite sign, since the resultingSeebeck thermo-induced voltage due to the electronic carrier will beopposite to the Seebeck voltage due to the ions. Hence, a polyanion(cation conductor) should be used when hole transport is considered andan polycation (anion conductor) should be used when the electrontransport is considered. If the ions and electronic carriers haveopposite sign, the resulting Seebeck may be reduced.

The at least one conducting composition may comprise P3PT-COOK.

P3PT-COOK refers to poly(3-carboxy-pentylthiophene).

Further, the conducting composition may comprise Nafion. Nafion is asulfonated tetrafluoroethylene based fluoropolymer-copolymer, known tothe skilled person.

Further, the conducting composition may comprise conducting polymer withpolyelectrolyte such as a blend of PEDOT and PSS; or P3PT-COOK,poly(4-2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl-methoxyl)-1-butanesulfonicacid) (PEDOT-S).

In embodiments of the first aspect, the conducting composition is anelectrolyte composition.

By the term an electrolyte, it is herein meant an ionically conductingmedium, such as a solution of an ionic compound. An electrolyte allowsion movement. Further, an electrolyte is normally electricallyisolative, i.e. it is not an electron conductive medium. In anelectrolyte an electric current is normally carried only by the movementof ions.

In embodiments of the first aspect, the conducting composition comprisesa polyelectrolyte, nanoparticles comprising ionic groups and/or an ionicliquid.

Thus, the electrolyte composition may comprise a polyelectrolyte,nanoparticles comprising ionic groups and/or an ionic liquid.

According to an embodiment, the first and/or second electrolytecomposition comprises a polymeric electrolyte, such as a polyelectrolyteor a polymer combined with a non-polymeric electrolyte.

As used herein, polyelectrolyte means an electrolyte comprising apolymer comprising anionic or cationic moieties.

Polyelectrolytes are polymers bearing an electrolyte group in theirrepeating units. Generally, the electrolyte groups can dissociate inaqueous solution such as water, making the polymers charged. They havethe properties both from electrolytes (salts) and polymers (highmolecular weight compounds), which can be also called polysalts.

Further a polyelectrolyte is an ionically conducting medium with arelatively high molecular weight such that the electrolyte has a lowdiffusion constant compared to the moveable ions, and thereby are lessmobile. In other words, the small ions (or mobile ions) should have alarger diffusion constant D_(mobile) [m²/s] compared to the diffusionconstant for the ions of the polyelectrolyte of opposite signD_(immobile) [m²/s]. Preferably, the ratio between the two diffusionconstants D_(mobile)/D_(immobile) is at least 10, or at least 100, or atleast 1000, or at least 10000 in a thermoelectric device according tothe invention, which is ready for use.

In general, the diffusion constant can be measured e.g. by adding amarker (such as an optical marker or by using elemental analysistechnique (such as NMR). There are plenty of optical markers (molecularfluorescent probes) that are charged dye molecules that can interact(via electrostatic attraction) either with the polyelectrolytes or thecharged ions. Different markers may be used for the mobile ions and forthe electrolyte, and one may thereby determine the different diffusionconstants using Fick's diffusion law.

Alternatively or additionally, for stationary or truly immobileelectrolytes it is only the mobile ions that is providing the electricalsignal. So, in that situation, it is possible to determine the diffusionconstant directly by a simple electrical method. Adding a highconcentration salt in one reservoir and following the increase inelectrical potential between the two electrodes is a direct way todetermine the diffusion of ion. This may be repeated for severalconcentrations and thereafter extract the diffusion coefficient of themobile ion.

For more information see e.g. Journal of the American Chemical Society[0002-7863] Umberger, J Q year: 1945 vol: 67 iss: 7 pages: 1099-1109, orElectrochimica Acta [0013-4686] Hayamizu, K year: 2001 vol: 46 iss:10-11 pages: 1475-1485 or CATION AND WATER DIFFUSION IN NAFION IONEXCHANGE MEMBRANES: INFLUENCE OF POLYMER STRUCTURE. Yeager, H. L.,Steck, A. 1981 Journal of the Electrochemical Society 128 (9), pp.1880-1884.

The polyelectrolyte may thus be a polyanion or a polycation. If forexample the device comprises a single leg between the first and thesecond electrodes, the leg may comprise a polyanion, such as PSSNa.

As discussed above, the electrolyte composition may preferably be apolyelectrolyte, as polyelectrolytes facilitates a prevention of thatboth cations and anions move in the same leg. A transport of bothcations and anions in the same leg is undesirable, as it may result incharge cancellation that would minimize the Seebeck voltage and preventsignificant redox reaction at the electrode. Thus, by using a negativelycharged polymer in a first leg, cation transport is facilitated, and theuse of a positively charged polymer in a second leg results in anenhanced anion transport.

Preferably, at least one of the first and second electrolytecompositions comprises cross-linked electrolytes. At humidity levelshigher than 80% RH, the polyelectrolyte films can lose their mechanicalintegrity and dissolve in the absorbed water. To prevent this, thepolyelectrolytes of the first and second electrolyte compositions maypreferably be cross-linked.

In more detail, selective ion transport in a single leg or in a firstleg out of several legs may hence be obtained by using a polymer capableof being anionic. Hence, the polymer backbone may comprise anionicmoieties stabilized by mobile cations. A polyanion is an electrolytewherein the ionic conductivity of anions is low compared to the ionicconductivity of cations. The polyanion is hence a polymer having theanionic moieties; this ion is immobile since too large to contribute toany ion transport itself. However, the anionic polymer significantlycontributes to the selective transport of cations, since a stationarynegatively charged path is created for the cations to move on. Thepolymers may for example be chosen from the group of polyacids or a saltof the polyacid (carboxylic, phosphoric, sulfonic), such aspolystyrenesulfonic acid (PSSH), poly(3-carboxy-pentylthiophene)(P3PT-COOH), or blends thereof.

Likewise, selective ion transport in a second leg may be obtained byusing a polymer capable of being cationic. Hence, the polymer backbonemay comprise cationic moieties stabilized by anions. A polycation is anelectrolyte wherein the ionic conductivity of cations is low compared tothe ionic conductivity of anions. The polycation is hence a polymerhaving the cationic moieties; this ion is an essentially stationary ion,since a polymer is too large to be able to move through the electrolytelegs. However, the cationic polymer contributes to the selectivetransport of anions, since a positively charged path is created for theanions to move on. In this case, the polymer may for example be selectedfrom the group of polymer with ammonium chloride in the pendant, such aspoly((2-dimethylamino)ethylmethacrylate)methyl chloride quaternary salt(PMADQUAT), poly(allylamine hydrochloride) (PAH),poly(N-(3-aminopropyl)-N-(4-vinylbenzyl)-N,N-dimethylammonium chloride)(PAVDMA), or blends thereof.

According to one example, the electrolyte composition comprises apolyelectrolyte, nanoparticles comprising ionic groups or an ionicliquid. Using an electrolyte composition being a polyelectrolyte or anionic liquid is advantageous as it allows the legs to be manufactured byprinting or casting.

According to one example the Seebeck coefficient of the electrolyte islarger than 0.1 mV/K, or lager than0.5 mV/K, or lager than 1 mV/K, orlager than 5 mV/K, or lager than 10 mV/K. Additionally or alternatively,the Seebeck coefficient of the electrolyte is lower than 1000 mV/K, orlower than 500 mV/K, or lower than 100 mV/K, or lower than 50 mV/K, orlower than 10 mV/K.

A polyelectrolyte may have a Seebeck coefficient between 1 mV/ and 100mV/K, or a Seebeck coefficient between 10 mV/K and 100 mV/K. The thermalconductivity may be between about 0.1 Wm⁻¹K⁻¹ and 1 Wm⁻¹K⁻¹ The figureof merit should preferably be larger than ZT=0.1 at room temperature(20-25° C.), more preferably larger than 0.3, more preferably largerthan 0.5, even more preferably larger than 0.7, and yet more preferablylarger than 0.8 at room temperature (20-25° C.). The ionic conductivityat room temperature should preferably be larger than 0.003 S/m.

Polyelectrolytes are commonly belonging to the group of chargedmembranes, or ion selective membrane or ion-exchange membranes. All ofthese membranes types are applicable in the thermoelectric device of thepresent invention.

By the term an ionic liquid, it is herein meant a salt in the liquidstate, comprising ions.

According to one example, the conducting composition is hydrated orwetted. By hydrating or wetting the legs with the electrolytecomposition, the transport of ions is eased as the electrolytecomposition increases the electrical conductivity. The inventors alsosurprisingly found that the thermoelectric Seebeck voltage (related tothe difference in temperature ΔT by a proportionality factor α calledthe Seebeck coefficient, or V=αΔT) is very high and it significantlyincreases with increased humidity indicating that the effect is due tomobile ions. Hence, an increase in both conductivity and Seebeckcoefficient leads to a large power factor.

The thermoelectric device of the present disclosure can also be seen asan ionic thermoelectric generator integrated to a supercapacitor in onedevice. With a temperature gradient ΔT>0, the ionic thermoelectric powercharges the supercapacitor. The charged supercapacitor can be dischargedto release electrical energy only if ΔT=0.

According to an embodiment the first and second electrodes comprise amaterial selected from high capacitance materials. By high capacitancematerials is meant herein materials typically used in the electrodes ofsupercapacitors, e.g. in electrostatic (double layer), faradic (pseudo)or hybrid supercapacitors. In an electrostatic supercapacitor (layer ofelectronic charge on the metal electrode balanced by a layer of ions inthe electrolyte) the electrodes should have a large surface area inorder to maximize the electric double layer capacitance. In a faradicsupercapacitor an electroactive surface layer is used. Suchsupercapacitor electrode materials typically have a specific capacitancein the range of 10 F/g to 1000 F/g. Thus, according to an embodimentfirst and second electrodes have a specific capacitance in the range of10 F/g to 1000 F/g.

Examples of high capacitance materials useful in the electrodes of thethermoelectric device of the first aspect include materials having alarge surface area and electrically conductive polymer compositionscapable of being reduced and/or oxidized. By large surface area is meantherein materials having a specific surface area in the range of 50 m²/gto 5000 m²/g.

There are three main classes of materials useful in the electrodes ofthe thermoelectric device of the first aspect, which can be used aloneand/or combined in mixtures or composites: (1) carbon materials withhigh specific surface area, for example activated carbon, carbonaerogels, carbon nanotubes, templated porous carbons, carbon nanofibresand graphene networks; (2) conducting polymers, for example polyaniline,polypyrrole and polyethylene dioxythiophene, and (3) metal oxides, forexample RuO₂, IrO₂, MnO₂, NiO, Co₂O₃, SnO₂, V₂O₅, and MoO.

According to an embodiment, the electrodes of the thermoelectric devicecomprise a carbon material having a specific surface area in the rangeof 50 m²/g to 5000 m²/g, for example activated carbon, carbon aerogels,carbon nanotubes, templated porous carbons, carbon nanofibres orgraphene networks. According to an embodiment, the electrodes of thethermoelectric device comprise a metal oxide, for example RuO₂, IrO₂,MnO₂, NiO, Co₂O₃, SnO₂, V₂O₅, or MoO. According to an embodiment, theelectrodes of the thermoelectric device comprise an electricallyconducting polymer.

In embodiments of the first aspect, the electrically conductive polymercompositions of the first and second electrodes comprise redox polymercompositions.

The conductive redox polymer composition may comprise PEDOT,poly(styrene sulphonate) (PSS), polyaniline and/or polypyrrole. Thus,the conductive polymer composition may comprise PEDOT-PSS, which mayrelease cations and accept cations.

The conductive polymer may also be PEDOT-ClO₄ or PEDOT-Tos, in which theanions (counterion) is small and can leave the polymer electrodes.

Advantageously, the electrode may be composed of a metal electrode suchas an Au electrode coated with the conductive redox polymer composition.

An alternative to the conductive redox polymer composition is highcapacitance electrodes with a large surface area. Such high capacitanceelectrodes may be of materials such as porous carbon electrode aerogel,carbon nanotube network or metal nanoparticles.

The conductive polymers may be reversibly reduced and/or oxidized.

Hence, according to one example, the first electrically conductivepolymer composition is capable of being reversibly reduced, and thesecond electrically conductive polymer composition is capable of beingreversibly oxidized, or vice versa. This is advantageous as it enablesto produce an electrical power when submitted to a temperature gradient(equivalent to charging a battery) and to produce an electrical powerwhen no temperature gradient is applied anymore (equivalent todischarging the battery). Hence, the device can be used advantageouslywith alternative heat sources (e.g. the sun with days and nights).Hence, those devices can provide more constant electrical power versustime in case of alternative heat source and respond better the demand ofhuman activities. Another aspect of this device is the possibility toalternately generate electric current, and generate a temperaturedifference (the equivalent to the battery discharge leads to a Peltiereffect). In other words the device may first be used for generatingelectric current, thereafter for generating a temperature difference andthereafter for generating electric current etc.

In embodiments of the first aspect, the device is further comprising atleast one ion reservoir at the junction between the conductingcomposition and the first and/or second electrodes.

The ion reservoirs may comprise mobile ions. It is advantageous to useionic reservoirs since it ensures a good ionic conductivity in theelectrolyte, such as in polyelectrolyte legs. Further, the use of ionicreservoirs prevents any shortage of mobile ions.

As an example, there may be a first ion reservoir at the junctionbetween the conducting composition and the first electrode, and theremay be a second ion reservoir at the junction between the conductingcomposition and the second electrode.

According to one example, the first and second ion reservoirs comprise asalt, a wetted salt or a salt solution, preferably comprising inorganicions. The inorganic ions should be monovalent, in other words carry onecharge, in order to have the ability be transported by the first andsecond electrolyte composition, respectively, in the first and secondleg, respectively, of the device.

As discussed above, by using an ion reservoir several advantages can beachieved. Firstly, the addition of a reservoir at or integrated witheach of the electrodes permits more ions to flow through thepolyelectrolyte legs. Lack of an ion reservoir, may result in a shortlife-time of the device as a continuous transport of ions is necessaryfor the production of continuous electric current. Secondly, the iontransport is essential for the electrochemical reaction to take place atthe electrodes. Hence, by using ion reservoirs the reduction andoxidation on the respective electrode can be ensured.

In embodiments of the invention, the ion reservoirs may comprise a solidsalt that slowly dissolves as ions thermodiffuse through the legs inorder to preserve the ion concentration in the ion connector. The ionicreservoirs may also comprise a hygroscopic material.

In embodiments of the first aspect of the invention, the device iscomprising more than one leg comprising conducting composition. As anexample, the device may comprise two legs, and ions, or ions andelectrons, may be transported between a connector to the first andsecond electrodes, respectively. This means that e.g. cations may betransported between the connector and the first electrode first via afirst electrolyte in a first leg, whereas anions may be transportedbetween the connector and the second electrode first via a secondelectrolyte in a second leg.

Thus, the applied temperature difference may be between the electrodesand the connector.

Ions that may be transported are for example chlorine ions (Cl⁻) andsodium ions (Na⁺). Thus, Cl⁻ may be transported in one leg and Na⁺ maybe transported in the other leg in a two-legged device.

As another example, negative ions that can carry an electronic chargecarriers, such as I₃ ⁻ may be used. The difference between a chlorineanion and an tri-iodine molecule I₃ ⁻ is that the latter is veryelectrochemically active. Those I₃ ⁻ anions may function as shuttles forelectrons and may release the electrons at a cold electrode to becomeI₂. Hence one leg could comprise such a shuttle of anions moving in apolycation membrane. For the other leg, an oxidized form of a conjugatedpolyelectrolyte like P3PT-COOK may be used. Accordingly, in embodimentsof the first aspect, there is provided a thermoelectric device forgenerating electric current comprising a first leg connected to thefirst electrode and a second leg connected to the second electrode,wherein the first and second legs are coupled via a connector, wherein

the first leg is connected to the first electrode by being in ioniccontact, the second leg is connected to the second electrode by being inionic contact, and the connector is in ionic contact with the first andthe second legs; wherein the connector comprises a compositioncomprising mobile cations and mobile anions

the device further comprises

-   -   a first ion reservoir being in ionic contact with the first leg,        and the first electrode and    -   a second ion reservoir being in ionic contact with the second        leg and the second electrode,

wherein the first and second ion reservoirs and the connector arespatially isolated from each other;

wherein the first leg comprises a first conducting composition beingcapable of transporting cations from the connector to the first ionreservoir, the second leg comprises a second conducting compositionbeing capable of transporting anions from the connector to the secondion reservoir;

and wherein

the first electrode comprises a layer of a first electrically conductivepolymer composition capable of being reduced which is in ionic contactwith the first ion reservoir, and the second electrode comprises a layerof a second electrically conductive polymer composition capable of beingoxidized which is in ionic contact with the second ion reservoir.

According to an embodiment, said first electrically conductive polymercomposition is capable of being reversibly oxidized, and said secondelectrically conductive polymer composition is capable of beingreversibly reduced.

Consequently, when a temperature gradient or a temperature difference isimposed between the electrode and the connector, the ions and electronsin the connector may thermo-diffuse through the legs comprising anelectrolyte composition towards the cold side and generates a measurablethermo-voltage.

Thus, the first and/or second conducting compositions may furthercapable of transporting electrons.

Further, the first and/or second conducting compositions may beelectrolyte compositions.

The electrochemically active electrode can be an inorganic material withion and electron conductivity (for instance inorganic materials used forelectrochromism); a conducting polymer with ion and electronconductivity; a non conducting polymer matrix comprisingelectrochemically active molecules or nanoparticles, dispersed in thenon conducting polymer matrix, providing electrical conductivity; a nonconducting polymer backbone with pendant group(s) that areelectrochemically active molecules providing electrical conductivity; orany combination thereof.

In order to increase the ionic conductivity in the conducting polymerelectrodes, an ionic channel of high conductivity in the electrodes maybe included.

The connector may comprise a composition of mobile cations and mobileanions. As used herein, mobile cations and anions means that whenapplying a temperature gradient, the cations will move from theconnector, through the first leg to the first ion reservoir and theanions will to move from the connector through the second leg to thesecond ion reservoir. The mobile cations and the mobile anions maypossibly also move in the opposite direction, from the ion reservoirs tothe connector, depending on the driving force. It is important that thematerial in the connector has a high conductivity for both anions andcations.

If the device is arranged to conduct both ions and electrons in the legor legs, the connector may preferably comprise a conducting polymersince such polymers may be capable of transporting both ions andelectrons.

Further, the connector may for example comprise a conducting polymer, ora charge transfer conducting organic salt dispersed in a medium thatconduct ions (for instance PEO) or a composite system made of “small”conductors, such as graphene or CNT or carbon black or metalnanoparticles, blended with an ion conductor material, such as PEO witha salt or any ion conducting matrix forming a interpenetrated networkensuring ions and electrons conduction.

The ion reservoirs may form a place where the transported cations andanions can be collected. The first ion reservoir may be in ionic contactwith the first leg, whereas the first electrode and a second ionreservoir may be in ionic contact with the second leg and the secondelectrode. The ion reservoirs can be arranged as an external storageplace for the transported ions. The ion reservoirs can also beintegrated in each leg at a distance from the connector or be integratedin each of the two electrodes.

The ion reservoirs may contain a hydroscopic material or a watersolution in order to be able to collect high amount of salt.

According to one example, the concentration difference between the tworeservoirs is initially low, preferably lower than 0.001%, or lower than0.01% or lower than 0.02% or lower than 0.03%, or lower than 0.05%, orlower than 0.1%, or lower than 0.5%, or lower than 1%, or lower than1.5%, or lower than 2%, or lower than 2.5%, or lower than 3%, or lowerthan 5%, or lower than 10%, with respect to the ions which are to betransported.

In embodiments of the invention, the first and/or second ion reservoirsand/or the connector further comprises a hygroscopic material

In a preferred embodiment of the invention, the location of the ionreservoir is optimized in order to achieve an efficient accumulation ofthe mobile ions in the ion reservoirs and avoiding salt deposition inother parts of the device. For example, in embodiments of the inventionthe first ion reservoir may be arranged in direct contact with the firstelectrode and in direct contact with the first leg, while there is aspace between the first electrode and the first leg. Thereby, the firstelectrode and first leg are in indirect ionic contact via the first ionreservoir. Likewise, the second ion reservoir may be arranged in directcontact with the second electrode and in direct contact with the secondleg, while there is a space between the second electrode and the secondleg. Thereby, the second electrode and second leg are in indirect ioniccontact via the second ion reservoir. This is beneficial since someconducting polymer electrode possesses a polyelectrolyte and thus have aselective ion motion, this would cause difficulties for the mobile ionto enter the electrode and/or difficulties for the counter ion to leavethe electrode. This would result in unfavorable salt deposition i.e. theformation of solid salt particles in the interface between theelectrodes and the legs which could hinder ionic transport. This couldcause the ionic thermoelectric effect to decrease or even stop.

The ion reservoirs should advantageously be arranged forming aninterface between each of the electrodes and each of the legs.

In more detail, the first electrolyte composition may comprise ananionic polymer; and/or the second electrolyte composition may comprisea cationic polymer. This is advantageous, as the ion transport can becontrolled so that charge cancellation, resulting from movement of bothanions and cations in the same leg, is avoided.

According to one example, the first and/or second electrolytecomposition is a composite made of the polyelectrolyte and a neutralmatrix favoring ionic motion. The neutral matrix can be a liquid solventor a solid solvent, such as succinonitrile or polyethyleneoxide.

According to one example, the electrically conductive polymercompositions of the first and second electrodes comprise redox polymercompositions. When the ions are transported from the ions connector toreservoir, a reduction of the redox polymer composition is induced atthe first electrode, while at the second electrode an oxidation reactionis reduced in the second electrode. Thereby, an electric potential andelectric current is achieved in an external circuit.

The hygroscopic material in the connector is advantageous when water isused as a solvent to wet the polyelectrolyte and the connector. Using ahygroscopic material help to more easily keep the water in the deviceand to prevent it from evaporating. Thereby, the water moleculesabsorbed in the hydroscopic material screen the charge of the ions bybuilding a solvation shell around each ion. As a result, theelectrostatic attraction between the mobile cations and anions weakensand the activation energy for the ion transport decreases. Increasingfurther the humidity leads to the creation of water percolation paths,the film is wet, and the ionic conductivity tends to saturate towardsthe ionic conductivity of the liquid phase. Therefore, the first and/orion reservoirs and/or the connector may also comprise a salt solution.Any known hygroscopic salt could be favorable. Any known hygroscopicsalt introduced in a polymer matrix could also be favorable. Anon-limiting example is NaCl in a polyethyleneoxide matrix.

According to one example, the connector may comprise a salt, a wettedsalt, a salt solution, salt in a polymer matrix and/or an ionic liquid,preferably comprising inorganic ions.

Alternatives to water as solvent are polar high boiling point solvents,such as propylene carbonate, DEG (diethylene glycol), PEG (polyethyleneglycol) or other non volatile materials such as ionic liquid,succinonitirle, as such or gellified with a polymer.

By using the principle of ionic transport instead of the conventionalelectric current principle, thermoelectric materials based on organicmaterials can be used. This provides the possibility for producing theassembly using printing techniques.

According to an embodiment said first electrode, second electrode, firstelectrolyte composition, second electrolyte composition and/or connectorcan be applied by liquid deposition techniques.

According to an embodiment the thermoelectric device is arranged on aflexible solid substrate.

In an embodiment, both the connector and the ion reservoirs may comprisemobile ions.

As discussed above, the device may further be adapted to generate atemperature difference as a response to an applied voltage between theelectrodes.

In embodiments of the invention, there is provided a thermoelectricdevice for generating a temperature difference comprising a first legconnected to the first electrode and a second leg connected to thesecond electrode, wherein the first and second legs are coupled via aconnector, wherein the first leg is connected to the first electrode bybeing in ionic contact, and the second leg is connected to the secondelectrode by being in ionic contact; the device further comprises

-   -   a first ion reservoir comprising mobile cations, being in ionic        contact with the first leg, and the first electrode and    -   a second ion reservoir comprising mobile anions, being in ionic        contact with the second leg and the second electrode,

wherein the first and second ion reservoirs and the connector arespatially isolated from each other; wherein the first leg comprises afirst conducting composition being capable of transporting cations fromthe first ion reservoir to the connector the second leg comprises asecond conducting composition being capable of transporting anions fromthe second ion reservoir to the connector;

wherein the connector comprises a cation and anion transportingcomposition in ionic contact with the first and the second legs; andwherein the first electrode comprises a layer of a first electricallyconductive polymer composition capable of being oxidized which is indirect contact with the first ion reservoir, and the second electrodecomprises a layer of a second electrically conductive polymercomposition capable of being reduced which is in direct contact with thesecond ion reservoir.

In an embodiment, both the connector and the ion reservoirs may comprisemobile ions.

According to an embodiment, said first electrically conductive polymercomposition is capable of being reversibly reduced, and said secondelectrically conductive polymer composition is capable of beingreversibly oxidized.

As discussed above, the first and/or second conducting compositions mayfurther capable of transporting electrons.

Further, the first and/or second conducting compositions may beelectrolyte compositions.

As discussed above, the first electrically conductive polymercomposition may be capable of being reversibly reduced, and wherein thesecond electrically conductive polymer composition is capable of beingreversibly oxidized.

Further, the first and second ion reservoirs may comprise a salt, awetted salt or a salt solution, preferably comprising inorganic ions.

Moreover, the connector may comprise a salt, a wetted salt, or a saltsolution, salt in polymer matrix, ionic liquid preferably comprisinginorganic ions.

As an example, the first and/or second ion reservoirs and/or theconnector further comprises a hydroscopic material.

Furthermore, the first conducting composition may comprise an anionicpolymer and/or the second conducting composition may comprise a cationicpolymer.

It is also to be understood that in the embodiments in which the devicecomprises a first and a second leg, the conducting composition maycomprise a polyelectrolyte, nanoparticles comprising ionic groups and/oran ionic liquid.

As an example, the first conducting composition may comprise a polymerselected from the group of polyacids or a salt of the polyacid, such aspolystyrenesulfonic acid (PSSH), poly(3-carboxy-pentylthiophene)(P3PT-COOH) or blends thereof.

Furthermore, the second conducting composition may comprise a polymerselected from the group of polymer with ammonium chloride in thependant, such as poly((2-dimethylamino)ethylmethacrylate)methyl chloridequaternary salt (PMADQUAT), poly(allylamine hydrochloride) (PAH),poly(N-(3-aminopropyl)-N-(4-vinylbenzyl)-N,N-dimethylammonium chloride)(PAVDMA) or blends thereof.

In embodiments of the first aspect, the conducting composition comprisesconducting polymer with polyelectrolyte such as a blend of PEDOT andPSS; or P3PT-COOK,poly(4-2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl-methoxyl)-1-butanesulfonicacid) (PEDOT-S).

In embodiments of the first aspect, the second conducting composition ishydrated or wetted.

In embodiments of the first aspect, the electrically conductive polymercompositions of the first and second electrodes comprises redox polymercompositions.

As previously discussed, conductive polymer composition may comprisePEDOT, poly(styrene sulphonate) (PSS), polyaniline and/or polypyrrole.

Further, the first and second ion reservoirs may comprise a salt, awetted salt or a salt solution, preferably comprising inorganic ions.

In embodiments, the connector comprises a salt, a wetted salt, or a saltsolution, salt in polymer matrix, ionic liquid preferably comprisinginorganic ions.

In embodiments, the first and/or second ion reservoirs and/or theconnector further comprises a hydroscopic material.

As a second aspect of the invention, there is provided a thermoelectricassembly comprising a thermoelectric device according to any aspectabove in which the device is comprising a first and a second leg, aconnector, a first and second ion reservoir, and further,

-   -   a third leg,    -   a fourth leg,    -   and an additional connector, wherein    -   the connector of the thermoelectric device is separated into a        first connector portion and a second connector portion, which        connector first and second connector portions are spatially        separated from each others;    -   the first leg of the thermoelectric device is connected to the        first connector portion;    -   the second leg of the thermoelectric device is connected to the        second connector portion;    -   a third leg ionically connecting the first connector portion and        the additional connector, which leg comprises a third conducting        composition being capable of transporting anions from the first        connector portion to the additional connector;    -   a fourth leg ionically connecting the second connector portion        and the additional connector, which leg comprises a fourth        conducting composition being capable of transporting cations        from the second connector portion to the first ion reservoir.

As a third aspect of the invention, there is provided a method forgenerating electric current comprising the steps of:

providing a thermoelectric device according to any aspect above, and

applying a temperature difference over the conducting composition.

Advantageously, the method for generating electric current according tothe present invention may be used in waste heat management, also whendealing with waste heat in warm fluids (50-250° C.), available inindustry, electricity production, buildings and transport. Also, whenconsidering renewable energy sources, such as solar cells, the methodmay be used to increase the efficiency by using heat that isconventionally lost.

In embodiments, the method is comprising,

-   -   providing a thermoelectric device or assembly as defined above        in which in which the device is comprising a first and a second        leg, a connector, a first and second ion reservoir, and    -   providing a first temperature in the connector and a second        temperature in the first and second ion reservoirs, wherein the        first temperature is lower than the second temperature.

As a fourth aspect of the invention, there is provided a method forgenerating a temperature difference comprising the steps of:

providing a thermoelectric device according to any aspect above, and

applying a potential difference between the electrodes.

In embodiments, the method is comprising

-   -   providing a thermoelectric device or assembly as defined above        in which the device is comprising a first and a second leg, a        connector, a first and second ion reservoir, and, and    -   applying a potential difference between the first and second        electrodes.

As a fifth aspect of the invention, there is provided the use of athermoelectric device or assembly according to any aspect above as atemperature sensor, or for charging a capacitor.

As a sixth aspect of the invention, there is provided the use of apolymeric composition capable of conducting both ions and electrons in athermoelectric device. According to an embodiment the polymericcomposition comprises a polymeric electrolyte. The polymeric electrolyteof the sixth aspect of the invention can be further defined as describedabove with reference to the previous embodiments.

As a seventh aspect of the invention, there is provided the use of aconductive polymer composition in the electrodes of a thermoelectricdevice. The conductive polymer composition of the seventh aspect of theinvention can be further defined as described above with reference tothe previous embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, wherein:

FIG. 1 shows a schematic drawing of a thermoelectric generator accordingto the invention.

FIG. 2 shows a schematic drawing of a thermoelectric cooler according tothe invention.

FIG. 3 shows a schematic drawing of a device used for measuring theexample devices.

FIG. 4 a shows the ion flow direction in vertical device.

FIG. 4 b shows the ion flow direction in lateral device.

FIG. 5 is a plot of open potential versus time.

FIGS. 6-7 are plots of ionic conductivity and ionic Seebeck coefficient,respectively.

FIG. 8-11 show different measurement results made on example devices.

FIG. 12 shows a schematic drawing of an embodiment of a multileggeddevice according to the invention.

FIGS. 13 a and 13 b show the chemical structure of PSSNa and PMDQUAT,respectively.

FIG. 14 shows a schematic drawing of some of the components of athermoelectric generator according to the invention.

FIGS. 15 a and 15 b show a further schematic drawing of some of thecomponents of a thermoelectric generator according to the invention(FIG. 15 a) and an example on a multilegged device (FIG. 15 b).

FIG. 16 shows a single-leg PSSNa ionic thermoelectric generator.

FIG. 17 shows electrically coupled single-leg PSSNa ionic thermoelectricgenerators.

FIG. 18 shows the evolution of the open-circuit potential in the singleleg PSSNa ionic thermoelectric generator.

FIG. 19 shows the evolution of the output voltage versus time for thesingle leg PSSNa ionic thermoelectric generator submitted to atemperature gradient ΔT=1.2 and connected to various load resistance(750 Ohms, 2 MOhms, 7.5 MOhms).

FIG. 20 shows the thermogenerated electrical power for the single legPSSNa device with different load resistance (ΔT=1.2). The solid blacksymbol corresponds to the same device connected subsequently to variousload resistance. The star symbols corresponds to different devices withdifferent load (ΔT=1.2) generated from the output voltage curves in FIG.19.

FIG. 21 shows the molecular structure of P3PT-COOK.

FIG. 22 shows the evolution of the total electrical conductivity(ionic+electronic), total Seebeck coefficient and power factor versus RHof a reference device based on two gold electrodes, with differentmaterial in the conducting leg.

FIG. 23 shows the power vs. time for a reference device based on twogold electrodes, with different material in the conducting leg.

FIG. 24 shows the electric power output vs. resistance load of aP3PT-COOK thermogenerator (one leg only) at different humidity levels.

FIGS. 25 a-25 c show thermoelectric Properties for PEDOT:PSSH-DEG,PEDOT:PSSH and PEDOT:PSSH-PSSNa at different RH: (a) δ; (b) S; (c) PF.

FIGS. 26 a-26 c show thermoelectric Properties for P3HT, P3PT-COOH andP3PT-COOK at different RH. (a) δ; (b) S; (c) PF.

FIG. 27 a shows the evolution of the ionic conductivity (a), Seebeckcoefficient (α) and corresponding power factor (σα²) for PSS:Na versusRH. The inset shows the chemical structure of PSS:Na.

FIG. 27 b shows the evolution of the thermal conductivity (λ), the powerfactor (ασ²) and ZT versus RH. All measurements are done at roomtemperature.

FIG. 28 a shows the charge-discharge curves recorded afterheating-cooling cycles (40 minutes).

FIG. 28 b shows the energy density versus temperature gradient.

FIG. 29 shows impedance spectroscopy data. Phase angle and Capacitancefor device, PEDOT:PSS/PSS:Na/PEDOT:PSS, with applied AC at 100 mV atsaturated water atmosphere.

DETAILED DESCRIPTION Schematic Description of a Thermoelectric Generatorwith a Single Leg

A schematic drawing of an thermoelectric device having a single leg isshown in FIG. 16. The single leg comprising a polyelectrolyte isarranged between two electrodes (PEDOT) comprising a conducting polymer.An applied temperature difference between the electrodes drives ions inthe single leg towards one of the electrodes (in this case the “upper”electrode in FIG. 16). In order to transform the ionic current generatedin the polyelectrolyte leg into an electronic current in the externalcircuit, an electrochemical reaction is required at the electrodes. Forthis purpose, a conducting polymer, more especially PEDOT-PSS, is usedas electrodes since such material transports both electronic and ioniccharge carriers. Importantly, a conducting polymer, typically oxidized,possesses a vanishingly small band gap and thus undergoes anelectrochemical reaction a very small applied voltage. For this reason,conducting polymers are used as electrode in supercapacitors. Here, weexpect that the electric potential difference between two electrodes dueto the thermo-diffusion of ions in the polyelectrolyte induces anelectrochemical reaction at the conducting polymer electrodes. For thepolyanion PSSNa, the cations thermo-diffuses to the cold PEDOT-PSSelectrode and increase its electrostatic potential, which triggers areduction of the conducting polymer with an electron coming from theexternal circuit:

PEDOT⁺PSS⁻ +e ⁻+Na⁺→PEDOT⁰+PSS⁻Na⁺

At the hot electrode, the oxidation of PEDOT-PSS is the oppositedirection of the chemical equation. Both reduction and oxidation arepossible at the PEDOT-PSS electrodes because the polymer is not fullyoxidized in its pristine conducting form. In order to ensure a goodionic conductivity in the polyelectrolyte legs, as well as no limitationdue to the amount of mobile cations, two reservoirs of equal NaCl saltconcentration are added at the junctions between the PEDOT-PSSelectrodes and the polyanion. The reservoirs are designed with PDMS andfilled in with an electrolytic gel made of 10% of polyethyleneoxide(Mw=100,000) and an aqueous solution of NaCl (1 M)

For some applications that do not require lot of current but rather ahigh voltage, it could be advantageous to couple single leg generatorselectrically, e.g. via a metal or a conducting polymer, see FIG. 17. Thesingle-leg ionic TEG may thus be connected thermally in parallel andelectrically in series by bridging the top-electrode of a single-legionic TEG with the bottom electrode of the adjacent single-leg ionic TEGvia electronic conductor (no need of ionic conduction) preferably of lowthermal conductivity and high electrical conductivity (e.g organicconductor such as PEDOT-Tos, polyaniline-CSA, charge transfer salt).

Such application could be for instance to switching a transistor byapplying a voltage via the thermoelectric voltage. The transistor isswitched OFF or ON depending on the temperature gradient.

Schematic Description of a Thermoelectric Generator with Two Legs

FIG. 1 shows a thermoelectric generator (TEG) (100) according to theinvention which comprises a first leg (101) connected to a firstelectrode (102) and a second leg (103) connected to a second electrode(104), wherein said first and second legs are coupled via a connector(105). In contrast to prior art, the TEG in FIG. 1 is based on theprinciple of ion transport. It is to be understood that this drawing isnon-limiting. For example, the ion reservoirs 106 and 107 may bepositioned anywhere as long as they are in ionic contact with the first101 and second leg 103, respectively.

When applying a temperature gradient, the cations will move from theconnector (105), through the first leg (101) to the first ion reservoir(106) and the anions will to move from the connector (105) through thesecond leg (103) to the second ion reservoir (107)

The first and second legs (101,103) comprise electrolyte compositions.The first leg (101) comprises a first electrolyte composition beingcapable of transporting cations from said connector (105) to said firstion reservoir (106). Said second leg (103) comprises a secondelectrolyte composition being capable of transporting anions from saidconnector (105) to said second ion reservoir (107). The electrolyte maybe based on a polyelectrolyte, an ionic liquid, a macromoleculefunctionalized with an ionic group or a nanoparticle functionalized withan ionic group.

The first and second electrodes (102,104) comprise a composition capableof transforming the ion transport into an electron transport. This isdone by using a first electrode (102) comprising a layer of a firstelectrochemically active material, more especially an electricallyconductive polymer composition capable of being reduced, which is inionic contact with said first ion reservoir (106). Further, said secondelectrode (104) comprises a layer of a second electrically conductivepolymer composition capable of being oxidized which is in ionic contactwith said second ion reservoir. It is critical that electrodecompositions are electrically conductive and electrochemically active.

According to one example, the electrodes may be composed of a metalelectrode such as an Au electrode coated with the conductive polymercomposition capable of being oxidized or the conductive polymercomposition capable of being reduced. Preferably, the first and secondelectrodes comprise redox polymer compositions, such as PEDOT, PSS,polypyrol and/or polyaniline.

The connector (105) comprises a composition of mobile cations and mobileanions. As used herein, mobile cations and anions means that whenapplying a temperature gradient, the cations will move from theconnector (105), through the first leg (101) to the first ion reservoir(106) and the anions will to move from the connector (105) through thesecond leg (103) to the second ion reservoir (107). The mobile cationsand the mobile anions may possibly also move in the opposite direction,from the ion reservoirs to the connector, depending on the drivingforce. It is important that the material in the connector has a highconductivity for both anions and cations.

The ion reservoirs (106,107) are used in the TEG in FIG. 1 in order toform a place where the transported cations and anions can be collected.The first ion reservoir (106) is in ionic contact with the first leg(101), and the first electrode and a second ion reservoir (107) is inionic contact with the second leg (103) and the second electrode. Theion reservoirs can be arranged as an external storage place for thetransported ions. The ion reservoirs can also be integrated in each legat a distance from the connector or be integrated in each of the twoelectrodes.

In order to achieve the desired ion transport, it is a requirement thatthat the first ion reservoir, the first electrode, the first leg, andthe connector are arranged to allow direct or indirect ion transportfrom the connector to the first ion reservoir via the first leg.Likewise, the second ion reservoir, the second electrode, the secondleg, and the connector are also arranged to allow direct or indirect iontransport from the connector to the second ion reservoir via the secondleg. Hence, said first and second legs are both in ionic contact withthe connector.

However, it should be noted that ion transport between the first andsecond ion reservoirs, between the first and second electrode, andbetween the first and second legs should be avoided since this wouldaffect the thermoelectric effect of the TEG. For the same reason, thefirst and second ion reservoirs (106,107) and said connector (105) arespatially isolated from each other. This allows for ion transportbetween the cold side (electrodes and ion reservoirs) and the hot side(the connector) of the thermoelectric generator.

In FIG. 1, the TEG produces an electric current when the device isexposed to a temperature gradient, where the temperature is higher inthe connector than in the electrodes and ion reservoirs. The heat flowis inducing a transport of ions where the ions inside the connector (hotside) tend to move towards the distal part of the legs (cold side). Thisis called the thermodiffusion or Soret effect. The temperature gradientis hence the driving-force for the ion transport. Therefore, ions willbe a transported from the connector to the ion reservoirs.

When an electrolyte composition in the first leg allowing more cationsthan anions to be transported is used in combination with an electrolytecomposition in the second leg allowing more anions than cations to betransported, this does in itself not result in a continuous iontransport as it would not be possible to maintain charge-balance in theion reservoir. The inventors have found a way to collect the transportedions in the ion reservoir and achieving charge-balance and at the sametime convert the ionic transport to an external electric current.

In FIG. 1, a first electrode capable of undergoing a reduction reactionand a second electrode capable of undergoing an oxidation reaction areused. In the reduction reaction anions can be produced and in theoxidation reaction cations and electrons are produced.

As an example, PEDOT may be reduced at one of the electrodes and animmobile polyanion PSS may be produced, wherefrom a cation may arrive tobalance. At the other electrode, PEDOT may be oxidized, due to thatanions may arrive close to the PEDOT-PSS electrode but may not go intoit since PSS is an immobile polyanion. Hence, instead, the anions maystay in the ion reservoir close to the electrode and the mobile cations,normally balancing the polyanion PSS in the electrode, may also moveinto the reservoir. The lack of cations in the electrode or the excessof PSS immobile polyanions may therefore lead to the oxidation.

The reduction and oxidation reactions are initiated due to thethermodiffusion, because in order for the transport to occur forprolonged times simultaneous production of counter-ions in theelectrodes is needed in order to balance the transported cationsentering the first ion reservoir and to balance anions entering thesecond reservoir. Hence, as the mobile cations arrive in the first ionreservoir, charge-balance is obtained by anions formed in the reductionreaction. Similarly, as the mobile anions arrive in the second ionreservoir, charge-balance is obtained by a cation formed in or releaseddue to the oxidation reaction. Thereby, a continuous flow of mobilecations and anions from the connector to each of the reservoirs,respectively, is created. Due to the oxidation and reduction reactions,an external electrical current may be produced in a connected electroniccircuit as the oxidation results in free electrons and as electrons areneeded for the reduction to take place.

The ion transport is a result of thermodiffusion initiated by thetemperature gradient present as described above. However, the degree ofion transport is further controlled by for example the ion size of thecounter ions in the electrolyte compositions, the ion size and charge ofthe ion reservoirs, and the structure of any polymers present in theelectrolyte compositions, such as the use of substituents, degree ofcross-linking, amount of ionic groups etc. This is because the maximumtransport rate is limited to the rate of which ions can be transportedfrom the reservoirs to the connector through the electrolyte compositionand the energy barriers for reduction and oxidation in the electrodes.Below, different ways of controlling the transport rate is discussed.

In detail, a requirement of achieving ion transport from the ionreservoirs is that the cations and anions of the ion reservoirs havesufficiently small ion radius to be able to move through the electrolytecompositions. Hence, the ion reservoirs should comprise mobile cationsand anions, respectively. Since a polymer composition in general is avery porous structure, inorganic ions, preferably monovalent inorganicions such as H⁺, Li⁺, Na⁺, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, may move through theelectrolyte compositions. Preferably, ions that are notelectrochemically active themselves should be used. Some organic ionsare also considered to be sufficiently mobile. However, organic ionshaving more than 15 carbon atoms, preferably not more than 10 carbonatoms, more preferably not more than 5 carbon atoms, would not berealistic to use, since they become too big and too slow to transport.

Preferably, the ion concentration in the connector may be about the sameas the ion concentration in the ion reservoirs.

It is also advantageous that the ions are easily leaving the connector.This is achieved by using salts which easily dissociate or which are ina more or less dissociated form. This can be facilitated by using ahydrated or wetted salt, a hygroscopic salt, which form solvated shellsaround the ions. Another way of obtaining freely movable ions is to usea solution of salt as ion reservoir. Another alternative is to use polarhigh boiling point solvents, such as propylene carbonate, diethlyeneglycol, DEG, PEG or other non volatile materials such as an ionicliquid, succinonitirle, as such or gellified with a polymer.

Further, the ionic contact between the reservoirs and the legs is ofimportance. In order to achieve high degree of contact, the ionreservoir may be integrated with the electrolyte composition of the leg.

Further, it is advantageous to provide the cationic and anionic polymerwith counter ions, which are small and have easily leaving groups tofacilitate the ion transport.

Further, the reactivity of the reduction and oxidation agents in theelectrodes will influence the transport rate. The electrode may compriseof electrochemically material that conduct both ions and electroniccharge carriers. The electrode has advantageously a small band gap inorder to undergo an electrochemical reaction for very small voltage,such as the thermo-voltage produced by Soret effect of ions in apolyelectrolyte. Hence, the electrode should undergo a thermo-inducedelectrochemical reaction

Furthermore, the ion transport may be significantly controlled by addingdifferent additives, for instance polar high boiling point solvents,such as propylene carbonate, diethlyene glycol, DEG, PEG or other nonvolatile materials such as an ionic liquid, succinonitirle, as such orgellified with a polymer, or zwitter ions, to the electrolytecomposition. The inventors have found that the ion transport wassignificantly enhanced by adding water to the electrolyte composition.Preferably, the water is added to the ion reservoirs and/or theconnector, and thereafter it may penetrate into the legs by osmosis. Theelectrolyte compositions in the legs may comprise up to more than 50% ofwater regarded to its volume. The addition of water to the electrolytecomposition results in that the ions are solvated. As a result, theelectrostatic attraction between the mobile ions and the polyionsweakens and the activation energy for the ion transport decreases.Increasing further the humidity leads to the creation of waterpercolation paths, the film is wet, and the ionic conductivity tends tosaturate towards the ionic conductivity of the liquid phase. Inpreferred embodiments of the invention, the first and/or secondelectrolyte composition is hydrated or wetted. Advantageously, ahygroscopic material can be included in the electrolyte compositions inorder to keep the leg hydrated/wetted for prolonged time. Alternatively,a solution of salt can be used as ion reservoirs and connector.

The present invention further relates to a thermoelectric device forproducing a temperature gradient or temperature difference.

FIG. 2 shows a thermoelectric device for producing a temperaturegradient or temperature difference (200) according to the inventionwhich comprises a first leg (201) connected to a first electrode (202)and a second leg (203) connected to a second electrode (204), whereinsaid first and second legs are coupled via a connector (205). Incontrast to prior art, the device in FIG. 2 is based on the sameprinciple as the thermoelectric device in FIG. 1, namely the utilizationof ion transport as charge carriers. Therefore, the device is built onthe same general concept as the TEG in FIG. 1 acceding to the inventionwith some modifications and differences explained below.

In analogy with the device described in FIG. 1, said first leg (201) isconnected to said first electrode (202) by being in ionic contact, andsaid second leg (203) is connected to said second electrode (204) bybeing in ionic contact. Further, the thermoelectric cooler alsocomprises a first and second ion reservoir where said first ionreservoir (206) is in ionic contact with said first leg (201) and saidfirst electrode and said second ion reservoir (207) is in ionic contactwith said second leg (203) and said second electrode. Furthermore, saidfirst and second ion reservoirs (206,207) and said connector (205) arespatially isolated from each other;

However, in the thermoelectric device in FIG. 2, the first ion reservoir(206) comprises mobile cations, and said second ion reservoir (207)comprises mobile anions which are transported from the ion reservoirs tothe connector via the first and second leg. Therefore, said first leg(201) comprises a first electrolyte composition being capable oftransporting cations from said first ion reservoir (206) to saidconnector (205), said second leg (203) comprises a second electrolytecomposition being capable of transporting anions from said second ionreservoir (207) to said connector (205). The mobile ions are thencollected in the connector of the device so that charge-balance isobtained. The connector therefore comprises a cation and aniontransporting composition in ionic contact with said first and saidsecond legs.

Another difference is that said first electrode (202) comprises a layerof a first electrically conductive polymer composition capable of beingoxidized which is in direct contact with said first ion reservoir (206),and said second electrode (204) comprises a layer of a secondelectrically conductive polymer composition capable of being reducedwhich is in direct contact with said second ion reservoir.

In FIG. 2, a voltage is applied over the electrodes, which initiates anoxidation reaction in the first electrode as the electrode becomesdepleted in electrons. Thereby, cations are produced in the first leg.Similarly, a reduction reaction takes place in the second leg due toexcess amount of electrons in the second electrode. Thereby anions areproduced in said second electrode. The oxidized electrode thereforeproduces an excess amount of cations and the reduced electrode producesan excess amount of negative ions. This results in the movement ofexcess ions through each leg of the device to the connector. Thedriving-force of the ion transport is to obtain charge-balance of thesystem. This is achieved since by the transport the excess cation andexcess anion are collected which results in charge neutralization.

If the potentials at the electrodes are reversed compared to a referencestate, the ion motion becomes the opposite in both legs. As the ions maytransport heat, an ion motion in the opposite direction implies aninverse heat flow compared to the heat flow in the reference state.

Schematic Description of a Thermoelectric Generator with Multiple Legs

FIG. 12 shows a multilegged thermoelectric assembly, i.e. athermoelectric assembly comprising more than one pair of legs. Themultilegged thermoelectric assembly is arranged as described in relationto the device above, except that there are two more connectors and twomore legs. The first leg is arranged of a cation transporting material,and ionically connects said first ion reservoir and a first one of saidconnectors; the second leg is arranged of an anion transportingmaterial, and ionically connects said first one of said connectors and asecond one of said connectors; the third leg is arranged of a cationtransporting material, and ionically connects said second one of saidconnectors and a third one of said connectors; the fourth leg isarranged of an anion transporting material, and ionically connects saidthird one of said connectors and said second ion reservoir.

When a temperature difference or a potential difference is applied, anelectric current or a temperature difference, respectively is producedan analogy with the description above. Providing a multilegged device isadvantageous, as a higher voltage can be produced by the sametemperature difference.

Naturally, the device may be extended by further connectors and pair oflegs

Dimensions

The thermoelectric device according to the present invention may havevarious dimensions.

The efficiency of a thermoelectric device depends on more factors thanonly the maximum ZT of a material. This is primarily due to thetemperature dependence of all the materials properties (ionicconductivity, a, ionic Seebeck coefficient, a, and thermal conductivity,A) that make up ZT(T) as a function of temperature.

A small letter “z” is used for the figure-of-merit of a thermoelectricdevice in order to distinguish it from the material's figure of meritZT=α²σ/λ. The maximum efficiency (η) of a thermoelectric device is usedto determine zT. Like all heat engines, the maximum power-generationefficiency of a thermoelectric generator is thermodynamically limited bythe Carnot efficiency (ΔT/Th). If the temperature is assumed to beindependent and n-type and p-type thermoelectric properties are matched(α, σ and κ), (an unrealistic approximation in many cases) the maximumdevice efficiency is given by Equation (1) with Z=z.

$\begin{matrix}{\eta = {\frac{\Delta \; T}{T_{h}} \cdot \frac{\sqrt{1 + {zT}} - 1}{\sqrt{1 + {zT}} + \frac{T_{c}}{T_{h}}}}} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

In order to obtain the maximum efficiency of the TEG, dimensions of thelegs need to be optimized such that the lengths L_(n) and L_(p) and thecross section areas S_(n) and S_(p) of the legs satisfy the Equation(2), wherein n stands for n-type and p stands for p-type. The lengthsL_(n) and L_(p) are the lengths of the legs extending from the cold sideto the hot side. The cross section areas S_(n) and S_(p) of the legs arethe cross section areas through which the ions are moving when theydiffuse by the temperature difference along the lengths of the legs.

$\begin{matrix}{\frac{l_{n}S_{p}}{l_{p}S_{n}} = ( \frac{\sigma_{n}\lambda_{n}}{\sigma_{p}\lambda_{P}} )} & {{Eq}.\mspace{14mu} (2)}\end{matrix}$

As far as the device architecture is concerned, miniaturization is knownto improve the efficiency of Peltier coolers (large heat flow).

In the following, reference is made to FIG. 14, which shows a schematicdrawing of some components of thermoelectric device 140 comprisingconnectors 145 ionically coupled via legs 141 and 143. The legs 141 and143 of the device may be provided as a film comprising the electrolytecomposition. The thickness of the film of electrolyte composition L inthe legs may vary. The thickness of the film, i.e. length L, may be inthe range of from 1 nm to 5 dm, for instance in the range of from 10 nmto 5 cm, or from 10 μm to 5 mm, such as in the range of from 1 nm to 500μm, as for example in the range of from 10 μm to 500 μm, such as in therange from 50 μm to 500 μm, or in the range of from 1 nm to 10 μm, suchas in the range from 1 nm to 1 μm.

If screen printing is used, the film of electrolyte composition, i.e.length L, may be in the range of from 1 μm to 500 μm. If spin coating isused, the film of electrolyte composition may be in the range of from 1nm to 10 μm. If liquid handling robot is used, the film of electrolytecomposition may be in the range of from 1 mm to 5 dm.

Preferably, the thermoelectric device may have dimensions of the legsallowing for relatively low resistance and relatively high power.

The electrolyte composition in the legs may have a thickness of about 1cm. The temperature difference over such a leg may be up to e.g. about100 K.

The electrolyte composition in the legs may be of a given materialhaving a thickness of about 50-100 μm. The temperature difference oversuch a leg may be up to e.g. about 30 K, given a temperature below 200°C. on the hot side.

The figure-of-merit may be in the range of from 0.1 to 2, or in therange of from 0.5 to 1.8, or in the range from 0.8 to 1.5. A relativelyhigh Seebeck voltage is preferable.

Both the dimension of the connector 145 and the dimension of the legs141, 143 influences the performance of the device.

If the connector has a relatively large length compared to itswidth/thickness, the resistance of the connector is very large since theions need to travel a long distance, which may limit the power of thedevice.

If the connector 145 has a length Lc in about the same range as itswidth/thickness, typically a length which is not more than 10 timeslarger than the width/thickness, more typically not more than 5 timeslarger than the width/thickness, the resistance of the connector 145 issmall, which is favorable in term of internal resistance.

However, considering a relatively thick connector 145 (Lc>>), typicallylarger than twice the thickness L of a leg 141, 143, also implies thatthere is a large temperature drop across the connector that cannot beused for thermoelectric generation. So, in a relatively thick connectorthe voltage will drop, which will decrease the power of the device.However, there might be areas of applications where this is acceptable.

Theoretically, there will be an optimal thickness of the connector 145depending on its ionic conductivity and thermal conductivity. The ratioof the distance between the legs (d_(int)) to the width of the legs (W)may preferably be less than 1. The thickness of the connector Lc maypreferably be in the same order of magnitude as the thickness of thelegs L (assuming the ionic conductivity and the thermal conductivity ofthe legs and connector are of the same order of magnitude).

The dimension of the conducting polymer electrodes will limit themaximum amount of charges Q (integrated current dQ/dt) that can begenerated by the ionic thermoelectric generator. The dimension of theconducting electrodes will fully limit given that the generated ionconcentration gradient, when running the device, is not the limitingphenomena thanks to ion reservoirs that are large enough.

The amount of charges that may be generated by a given temperaturegradient before the device must be recharged and the process must bereversed, increases with the thickness of the PEDOT-PSS electrodes.Hence, it is advantageous to consider geometries where the conductingpolymer electrode has a large volume, but still a close distance to thelegs in order to avoid a large ionic resistance in the reservoir.

Possible architecture includes for instance a conducting polymerelectrode surrounding the electrolytic reservoir, with an additionalinsulating layer to avoid contact with the legs.

Vertical Versus Lateral Structures

The thermoelectric device according to the invention may be a verticaldevices or a lateral device. In a lateral device, FIG. 4 a, theconnector is normally arranged to the side of the reservoirs, and themobile ions move in a lateral direction, substantially parallel with thesurface area of the leg. In a vertical device, FIG. 4 b, the connector,leg and reservoir normally at least partially covers each other, and thesurface area of the electrolyte film makes up the cross section of thedevice. In more detail, in a vertical device the connector, leg andreservoir are normally arranged in substantially in the same plane on asubstrate, and the ion flows in a direction substantially parallel withsaid substrate. Further, in a vertical device the connector, leg andreservoir are normally arranged on top of each other on a substrate,each element in a different plane, and the ions flow in a directionsubstantially normal to said substrate.

The ionic thermogenerator demonstrated in the examples is fabricated asproof of concept. It is not intended to show the design optimum to getthe maximum power out of the device.

In the device presented in the examples, the length of the legs areabout L=1 cm, the thickness about T=150 microns, and the width about W=1mm. Because it is a lateral device, the cross section for the ioncurrent (W*T) is little, the length is long and thus the resistance ishigh.

Thus, for a conductivity of about 1 S/m, the resistanceR=1/conductivity*L/(W*T) will be about 0.1 MOhms. Assuming theresistance of the legs is the largest resistance in the device, fordevice with 2 legs, the internal resistance is thus 0.2 MOhms.

The maximum power of the device is obtained when the load resistance Ris similar to internal resistance Rint: Pmax=Vload2/Rint.=(Voc/2)2/Rint.The load voltage across the load resistance Vload is half theopen-circuit voltage Voc when the load resistance is equal to theinternal resistance. The open circuit voltage can be simply estimated bythe Seebeck coefficient of the two legs and the temperature gradientused. The sum of the Seebeck coefficient for the polycation andpolyanion legs is of the order of 50 mV/K. So, assuming ΔT=1 degree, theopen circuit voltage is 50 mV, and the load voltage is 25 mV. Themaximum power is 25²*10⁻¹¹ W=6.25 nW.

In another example, a device is constructed vertically (the temperaturegradient across the thickness of the polyelectrolyte film) with thefollowing dimensions of the legs: L=10⁻⁴ m, W=10⁻³ m, T=10⁻² m. In thiscase, the cross-section area, S, is given by W times T, and the lengthis given by L.

Given, an ionic conductivity of 1 S/m, the resistance of the leg is R=10Ohms. Hence, simply by going to a vertical architecture, the resistanceof the leg decreases by 4 orders of magnitude. Assuming the sametemperature gradient, the maximum power is then 62.5 μW.

In order to further increase the power for the same temperaturegradient, the number of legs in the thermoelectric modules may beincreased. N legs connected in series in a thermoelectric module willincrease the open circuit voltage by N times, but the internalresistance will also increase by N times. Thus, the maximum will alsoincrease by N times.

The limitation for a vertical architecture is in the achievabletemperature gradient. A thin film will lead to a small temperaturegradient, thus a small Seebeck voltage and small Voc and small poweroutput.

However for some applications, it is desirable to have thin devices,such as to put on the body (flexible) and use the heat from the body togenerate electricity to power some other devices.

Thicknesses in the sub-microns are possible to use for high ZT materialssuch as for applications in nanoelectronics.

Most of the applications envisaged will consider a polyelectrolyte legwith a thickness of 1 micron or larger. A typical thickness of apolyelectrolyte leg is in the range of from 10 to 1000 microns.

Manufacturing Techniques

In term of manufacturing, the advantage of using a polyelectrolyte isthe ability to process it starting from a solution. Hence, low costmanufacturing technique such as printing can be used. Screen printingtechnique is ideal to create patterns of a thickness in the range offrom 1 micron to 500 micron.

In more detail, the manufacturing may be performed by means of atechnique selected from a group comprising screen printing, wire-barcoating, knife coating, bar coating, spin coating, dip coating or spraycoating. This is advantageous as it normally allows for shortmanufacturing times.

Legs of a thickness in the range larger than 1 mm might be needed forsome specific applications using large temperature gradient, reachingthe upper limit of what can stand the materials (max temperature 300°C.). Thicker legs can be envisaged in application with a cold side atlow temperature, such as in airplane, where the temperature at 10 kmaltitude is −70° C. and the temperature in the plane or close to themotor is from room temperature to several hundred degrees Celsius.

In order to fabricate thick legs, in the range of from 1 mm to 10 dm,liquid handling robot can be used to fill-in plastic cavities withpolyelectrolytes. It is not excluded that the polyelectrolyte can beblended with more conventional plastics such that standard manufacturingtechniques for plastic, like extrusion or injection, are used.

If spin coating is used, the film of electrolyte composition may be inthe range of from 1 nm to 10 μm.

FIG. 15 a further illustrates how the components of a thermoelectricdevice 150 according to the present disclosure may be positioned. Theelectrode 154, reservoir 157, legs 151 and 152 and a connector 155 ofthe device is illustrated in FIG. 15 a. It may be advantageous toconsider geometries where the conducting polymer electrode 154 has alarge volume, but still a close distance to the legs 151, 152 in orderto avoid a large ionic resistance in the reservoir 157. Possiblearchitecture includes for instance a conducting polymer electrode 154surrounding the electrolytic reservoir 157, with a additional insulatinglayer 158 to avoid contact with the legs 152.

For a multi-legs device, the conducting polymer electrode can beconsidered up to 10 times thicker than overall thickness of the“leg+connector”, i.e. Z_(polymer) may be up to 10 times thicker thanZ_(a). However, the conducting polymer electrode may also be up to about100 times the thickness of the “leg+connector”

A multi-legged device is further illustrated in FIG. 15 b, in whichseveral connectors 155 and legs 151, 152 are coupled in series to anelectrode 157. This device also comprises an insulator layer 155.

EXPERIMENTAL EXAMPLES Example 1a Thermoelectric Properties ofPolyelectrolytes

The thermoelectric properties of the polyanion poly(styrene sulfonate)(PSS) with mobile sodium cations (Na⁺) and the polycationpoly-2-[(methacryloyloxy)-ethyl]trimethylammonium (PMADQUAT) with mobilechloride anions (Cl⁻) are measured in the device illustrated in FIG. 3.This device can be considered as the elementary power generator for apolyelectrolyte.

A glass substrate 352 with two pre-patterned gold electrodes 353, 354 bythermal evaporation (1 mm in width, 53 mm in length, approx. 100 nm inthickness for each and 1 mm apart from each other). Solutionpolyelectrolytes 351 (PSSNa or PMADQUAT, 2 wt % in distilled (DI) water,40 μl) were drop-casted on the prepared substrate and dried naturally.The obtained films give the thickness as 1.66 μm for PSSNa and 1.16 μmfor PMADQUAT.

A temperature difference is then applied between the two gold electrodesby a heater 355 and cooler 366 positioned below the glass substrate. Anelectric potential can be measured between the two gold electrodes.

Without temperature gradient the potential between the two goldelectrodes is small and fluctuates, see also FIG. 5. Upon heating, atemperature difference arises and the open circuit voltage, V_(OC),increases with time, and stabilizes at well defined values: V_(oc)=24.84mV/K for PSSNa and 1.96 mV/K for PMADQUAT when the measurement isperformed at a relative humidity of 50% RH. The Seebeck coefficient isdefined as the measured open circuit voltage, V_(OC), divided by thetemperature difference at the two gold electrodes, ΔT_(Au).

By the term relative humidity, it is herein meant the ratio of theactual amount of water vapor (absolute humidity) present in the air tothe saturation point at the same temperature.

The ionic conductivity, σ, and ionic Seebeck coefficient, α, can besystematically measured for different values of relative humidity. Theionic conductivity is typically low in dry films (for instance, ˜10⁻³S/m at 10% RH) and increases drastically up to 0.74 S/m for PSSNa and5.57 S/m for PMADQUAT at 80% RH, as can be seen in FIGS. 6 and 7,respectively.

When a temperature gradient, ΔT_(Au), of 1.2 K is imposed between thetwo gold electrodes coated by the polyelectrolyte films, the mobile ionsare expected to thermo-diffuse towards the cold electrode and generate ameasurable thermo-voltage. For humid films, it is clear that the sign ofthe mobile charged ions dictates the sign of the thermo-voltage. Thepolyanion PSSNa possesses a positive ionic Seebeck coefficient of +50mV/K at 80% RH; while the polycation PMADQUAT displays a negative ionicthermopower of −9 mV/K at 80% RH. Both the thermovoltage and theconductivity increase with the humidity, which supports the directinvolvement of mobile ions in the thermo-voltage.

Compared to electronic thermoelectrics, the power factors (σα²) of theseionic thermoelectrics are surprisingly high: 1830 pWm⁻¹K⁻² for PSSNa and410 μWK⁻² m⁻¹ for PMADQUAT at 80% RH. Assuming a thermal conductivity ofλ=0.3 WK-1 m−1 as typical for a polymer gel, thermoelectricfigure-of-merit (ZT) is 1.8 for PSSNa and 0.4 for PMADQUAT, equivalentto the best electronic thermoelectric materials.

Example 1b Electric Power Generation from One Leg Device or fromMultiple Single-Leg Devices

The open-circuit voltage of the device increases linearly with thetemperature gradient (FIG. 18) and its value is about 55 mV for 1 K,which is close to the measured ionic Seebeck coefficient with the Auelectrodes

The device is then connected to a load resistance and the output voltageacross the load is followed versus time. At the origin of the time axis,there is no temperature difference, but a temperature gradient isincreased until it reaches a constant value of ΔT=1.2 K at about 1500seconds. The initial output voltages are smaller than the open-circuitvoltage, as expected for a generator connected to a load resistance, butit increases steadily to become larger than the open-circuit voltage toreach a maximum at 64 mV (R=7.5 MOhms), 53 mV (R=2 MOhms), 32 mV (750kOhms). This increase in the output voltage corresponds to an inducedthermo-generated current of 8.53 nA, 26.5 nA, and 42.6 nA and a maximumelectrical power of 0.546 nW, 1.40 nW and 1.36 nW, for respectivelyR=7.5 MOhms, 2 MOhms and 750 kOhms. The total charge storedelectrochemically in the two PEDOT-PSS electrodes is Q=0.0001 coulombs.Like any electric power generator, the power output depends on the loadresistance and possesses a maximum when the load resistance is equal tothe internal resistance, here about 2.5 MOhms (FIG. 20.

Example 2 Point of the Electrochemically Active Electrodes

The strategy to increase the thermo-voltage is to connect polycation andpolyanion legs electrically in series and thermally in parallel, sincethey have Seebeck voltages of opposite sign. PSSNa has a positive ionicSeebeck coefficient, α, while PMADQUAT shows a negative Seebeck voltageat high humidity level. PSSNa may be defined as a P-leg and PMADQUAT asa N-leg.

Device 4, arranged generally as described in relation to FIG. 1, and inmore detail a connector to conduct ions comprising an aqueous solutionof NaCl, two ion reservoirs comprising a NaCl solution of the sameconcentration as the solution in the connector, a first and a secondleg, respectively, and electrochemically active PEDOT-PSS electrodes.The reservoirs are in contact with the legs and the electrodes.

Onto a glass substrate, two PEDOT:PSS electrodes are prepared bydrop-casting the solution and baked at 50° C. (L: 18 mm, W: 15 mm and T:8.6 um). PSSNa and PMADQUAT legs are fabricated with the their solution(2 wt % in DI water mixed with Silquest-187A silane (5 wt %)) and bakedat 110° C. for 5 minutes, with the help of sticky tape frame. Then, thesticky tapes are removed. Continuously, frames for the ionic conductor(L:11 mm and W: 6 mm) and reservoir (L: 38 mm and W: 5 mm) arefabricated thermally cross linking with SU-8 by using modes and baked at100° C. for 4 hours. The resulting frames have the thickness as around500 μm. Each device has the channels 1-mm-wide and 11-mm-long.

The edge electrodes of Device 4 are connected to a load resistance (50kOhms) when a temperature gradient of 1.2° C. is applied. The outputvoltage measured over the resistance is recorded versus time. The Device4 shows an increase in potential versus time upon applying thetemperature difference. A true electrical power is generated from thetemperature difference. The potential reaches a plateau indicative thatthe temperature gradient is now constant. In this specific experiment,the output voltage suddenly drops after 4500 s because the reservoirsare dried due to that the water of the NaCl solution has evaporated atthe hot side.

In Device 4, the temperature difference per mm is 1.2 K, and the Voltageoutput is illustrated in FIG. 8.

Example 3 Improvement of the Connector and Reservoir

The Device 5 is fabricated in the same way as Device 4 but the reservoirand ionic connectors are composed of a gel comprising NaCl 1M with 10%polyethyleneoxide (PEO). The presence of polyethyleneoxide in the gelslow down the evaporation of water and the output potential can bemaintained and recorded for a longer time than for Device 4 (given thesame load resistance and the same applied temperature difference), as isillustrated in FIG. 9.

Any power generator has its own internal resistance R_(in). When theload resistance is equal to the internal resistance, the power of thegenerator is maximum. This is observed also for our device, see alsoFIG. 10.

Example 4 Charge and Discharge

The output voltage over a resistance of 50 kOhm versus time may befollowed in a series of three cycles, H1+C1, H2+C2 and H3+C3,respectively, of charge (ΔT=1.2 C) and discharge (ΔT=0 C), illustratedin FIG. 11. The three heating-cooling cycles (obtained for Device 5 inN₂ atmosphere with 80% RH) are explained in detail below:

Heating half-cycle: Apply a temperature difference of ΔT=1.2 C betweenthe electrodes, connect the device with the load resistance and recordthe output voltage (H1 for 10 min, H2 for 20 min, H3 for 30 min).

Cooling half cycle: Stop heating the device and disconnect the out loadR. When the temperature difference ΔT between the electrodes gets to 0,connect the load and record the output voltage for some time (C1 for 10min, C2 for 20 min, C3 for 30 min).

For each cycle, the out load is only connected to the device whentemperature difference ΔT between the electrodes is in equilibrium.

When ΔT=1.2 C, an output power is measured, electrical current isgenerated. This leads to an electrochemical reaction in the PEDOT-PSSelectrodes: one is reduced, one is oxidized. As a result, the twoPEDOT-PSS electrodes are not at the same electrical potential even if notemperature gradient is applied. In other words, the heat charges aPEDOT-PSS battery cell.

When ΔT=0 C, there is still an output potential across the loadresistance since the two PEDOT-PSS electrodes are not at the samepotential. But this potential drops during time indicating that acurrent discharge is measured. When this current is zero, the twoPEDOT-PSS electrodes have the same oxidation level. This is equivalentto discharge a PEDOT battery.

When a temperature difference is applied to the device, the twoelectrodes undergo reduction and oxidation, respectively. If PEDOT isused, the PEDOT at one electrode becomes more reduced than the pristinePEDOT at the same electrode was, and the PEDOT at the other electrodebecomes more oxidized than the pristine PEDOT at that electrode was. Ifthe temperature difference is no longer applied, there is still anelectric potential difference between the electrodes, in other words acharged battery. If a resistance is connected to the PEDOT electrodes, adischarge current will flow through the resistance and the ions willmove in opposite direction inside the legs compared to when thetemperature difference was applied.

The amount of charges that may be stored in the electrodes of thisdevice is related to the capacitance of the electrodes and the thicknessof the PEDOT-PSS layer among others.

Example 5 Multiple Legs Ionic Thermogenerators

The voltage increases with the size of the electrolyte films in thelegs. Therefore, the voltage may increase by adding a number of legs.The power also increases with the size.

As an example, 1 cm² may correspond to about 1 V. If one leg correspondsto 1 μV, then a million of legs would correspond to 1 V.

In Device 6, which is a device arranged as described in relation to FIG.12, and manufactured in an analogous manner as described above, thetemperature difference per mm is 1.5 K.

For a multi-legs device, the conducting polymer electrode can beconsidered up to at least 10 times thicker than the overall thickness ofthe leg and the connector. For such large dimension of the electrode,the ionic resistance in the electrode will be limiting the internalresistance.

In term of power generation, a too large electrode will lead to a lowpower, but a larger total amount of charge generated, that is a longertime per a cycle.

The multiple legged device may be charged slowly, and dischargedrapidly.

For applications requiring higher peak power, a thermoelectric generatorslowly charging a supercapacitor, which can then deliver a large currentor peak power, is a solution.

Example 6 Reduction and Oxidation of PEDOT-PSS Electrodes

In general, a heat flow caused by a temperature difference between aso-called cold and hot part of the device, respectively, is inducing atransport of Na⁺ and Cl⁻ ions where the ions inside the connector (hotside) tend to move towards the distal part of the legs (cold side) inrelation to the connector.

The Na⁺ tend to move through the first leg and the Cl⁻ tend to movethrough the second leg. The sodium ions tend to move through the firstleg as the first leg comprises an immobile anionic polymer whichconstitutes a negatively charged path on which the ions can move on. Thechloride ions tend to move through the second leg as the second legcomprises an immobile cationic polymer which constitutes a positivelycharged path on which Cl— can be transported.

As Na⁺ enters an ion reservoir, situated at the distal part of the firstleg, there will be a driving force for obtaining charge balance whichcauses a reduction reaction at the first electrode. The first electrodeis composed of PEDO⁺PSS⁻ and the reduction reaction which occurs is:

PEDO⁺PSS⁻ +e−PEDOT⁰+PSS⁻

During the reduction reaction, a PSS⁻ is released and transported to thefirst ion reservoir to provide for charge-balance in the first ionreservoir where the Na⁺ ion has entered due to thermodiffusion.

The second electrode is composed of PEDOT⁰. As the Cl⁻ enters an ionreservoir, situated at the distal part of the second leg, there will bea driving force for an oxidation to occur at the second electrode:

PEDOT⁰→PEDOT⁺ +e ⁻

In more detail, the reaction in the second electrode can be described asfollows:

PEDOT⁰+PSSNa→PEDOT+PSS⁻ +e ⁻+Na⁺

In this case the Na⁺ ion is released and transported to the second ionreservoir to provide for charge-neutralization as Cl⁻ is entering thesecond ion reservoir.

By the oxidation reaction an electron is transported in an externalcircuit, built up between the two electrodes, from the second electrodeto the first electrode where the reduction reaction occurs.

Example 7 Combined Electronic and Ionic Charge Carriers

The inventors have found that it is possible to combine ionic chargecarriers and electronic charge carriers with conducting polymers andconjugated polyelectrolytes. Conjugated polyelectrolytes, such asP3PT-COOK shown in FIG. 21, are conjugated polymers that carry ionicgroups on their main chain (e.g. a carboxylate COO⁻K⁺). Conjugatedpolyelectrolytes can be electronically conducting upon oxidation(doping) and are intrinsically ionic conductors. If the mobile ions andelectronic charge carriers have the same sign, the total Seebeckcoefficient is presumably larger than the individual contributions: theelectronic Seebeck coefficient α_(elect) and the ionic Seebeckcoefficient α_(ion). If α_(elect) and α_(ion) have opposite sign, theSeebeck effects may partially cancel each other. Hence, even by usingmetal electrodes (ion blocking), the effect can be beneficial.

Preliminary experimental results on non-oxidized (undoped) P3PT-COOKreveals clearly that the important contribution from α_(ion) increaseswith the relative humidity as polyelectrolytes (FIG. 22) and may beresponsible for the large power factor (500 μWK⁻² m⁻¹). In that sample,the electronic contribution to the electrical conductivity is small dueto the very low oxidation level (10⁻⁵ S/cm). There may be an optimumoxidation level where the material is both a good electronic and ionicconductors.

Experiments were conducted with a simple thermoelectric generator: asingle leg of the thermoelectric materials connected to two electrodes

The first reference device was based on two gold electrodes connecting anon-conjugated polyelectrolyte PSSNa leg (as explained in the previoussections of the patent application). When a load is connected betweenthe two electrodes, the electric power decreases abruptly with time (seeFIG. 23 curve triangles). Since the Au electrodes are ions blocking, thecurrent transported is equivalent to a charging current for a capacitor.

AS a reference, the same experiment was conducted with P3PT-COOK and Auelectrodes. Interestingly the power decreases slowly versus time, whichmay be because there is a constant electronic current passing throughthe device, while the ionic current drops with time since K⁺ cationsaccumulate at the Au electrode surface (see FIG. 23, squares). Hence,the having a conducting composition comprising both electric chargecarriers and ionic charge carriers may extend the life-time ofthermoelectric generator; thereby providing more electrical power beforehaving to “discharge” the IOTEGs (to recover the pristine state of thedevice).

Further, experiments were conducted using the same setup as above butwith conducting polymer electrodes PEDOT-PSS in the electrodes in orderto allow electrochemical reaction and transduction of ions current intoelectronic current. Thus, the device had P3PT-COOK leg connected toelectrodes comprising PEDOT-PSS. In that case, the current is almostconstant (see FIG. 23), thus further indicating the beneficial effect ofcombining conjugated polyelectrolyte legs with the conducting polymerelectrodes.

In order to build a thermogenerators that combine the ionic andelectronic charge carriers, the connector (as mentioned for the purelyionic TEG) may comprise a conducting polymer since they can transportboth type of charge carriers.

Example 8 Single Thermogenerator with P3PT-COOK

The thermoelectric power factor PF=σS² increases with humidity andreaches 515 μWm⁻¹K⁻² at 80% RH. Assuming the thermal conductivity as0.35 W/mK, analogous to the value for polymers, resulting in ZT to bearound 0.52 at room temperature. To check this prediction of maximumpower factor at high humidity, one single thermoelectric leg composed oftwo gold contacts and a layer of P3PT-COOK was built. A temperaturegradient of 1.2 K was applied between the two gold electrodes and theywere connected to a load resistance. The electrical power of the singledevice was measured with different load resistances at different RHlevels (see FIG. 24). For each specified RH, electrical power reaches amaximum and then drops. The maximum electrical power output occurs in acertain range of load resistances which probably corresponds to theinner bulk resistance of the devices. Moreover, it is interesting tonote, that the electrical power increases vs RH by the same order ofmagnitude, as the increase of PF measured from two isolated measurementsof 5 and S.

Example 9 Thermoelectric Properties of Conjugated Polyelectrolytes andConducting Polymers

The thermoelectric properties of σ, S and PF were measured for

-   -   (1) conjugated polymer-polyelectrolyte: such as        poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate acid)        (PEDOT:PSSH). Here the conjugated polymer is oxidized (hence        called “conducting polymer”), so it is transporting very well        the electronic charge carriers.    -   (2) conjugated polyelectrolytes: such as the weak electrolyte        poly[3-(5-carboxypentyl)thiophene-2,5-diyl] (P3PT-COOH) and the        “salt” poly[3-(potassium-4-petanoate)thiophene-2,5-diyl]        (P3PT-COOK). Here the conjugated polymer (polythiophene) is very        little oxidized, so it does not conduct well the electronic        charge carriers.        For the sake of comparison with the first class, we also        characterized an oxidized conjugated polymer that does not        possess as counterions a polyelectrolyte (typically polyanionic)        but a small amion: tosylate. This conducting polymer is called:        poly(3,4-ethylenedioxythiophene):Tosylate (PEDOT:Tos).        For the sake of comparison with the second class of materials,        we have characterized a conjugated polymer that is not oxidized        and does not carry any ionic groups. This polymer is        poly(3-hexylthiophene) (P3HT).        The films were obtained by drop-casting the solution on a glass        substrate with two pre-patterned gold electrodes with the        geometry as 1 mm in width, 53 mm in length, approximate 100 nm        in thickness and 1 mm apart from each other. The electrical        conductivity a was calculated by using the equation σ=d/(R·A),        where d, R and A stand for the distance between two electrodes,        impedance at different frequency and the cross sectional area of        the film, respectively. The impedance measurement of the films        was carried out by using an Alpha high-resolution dielectric        analyzer with applied ac voltage (0.1 V) sweeping from 1 MHz to        5 Hz. The S measurement was performed by mounting the sample on        a thermoelectric heater-cooler pair. Voltage difference (ΔV)        between two electrodes was recorded by nanovoltmeter        (Kethley-1282A). The Seebeck coefficient was defined as        S=−ΔV/ΔT. In order to activate the ions, all measurements were        conducted inside a dessicator, where the relative humidity (RH)        levels have been controlled.

Results for Conjugated Polymer and Polyelectrolyte

Results are shown in FIGS. 25 and 26, as well as in Tables 1 and 2 below

(a) PEDOT:PSS

For the pristine PEDOT:PSSH, the electrical conductivity slightlydecreases vs RH indicating the detrimental effect of water on theelectornic transport. But up to 60% RH, it increases a little, which canbe ascribed to the small increase in the ionic δ that starts to bevisible and a bit larger than the electronic δ. The ionic S continuouslyincreases adding to the electronic S and become at least 10 timeslarger.

(b) PEDOT:PSS with DEG

In the chemically prepared PEDOT:PSSH water emulsion, there is an excessof the electronic insulating PSSH. A way to demix it and gather thisexcess of insulating materials in domains is by using high boiling pointsolvent that allows changing the morphology and creating athree-dimensional network of highly conducting polymer PEDOT:PSS. Thesolvent used to trigger the phase separation is diethylene glycol (DEG,2 wt %), DEG acts as a “secondary dopant”, which only changes themorphology but not the actual oxidation level. Hence, the samplePEDOT:PSSH-DEG has higher electronic conductivity than the pristinePEDOT:PSSH (FIG. 25 a). Increasing RH does actually decrease theelectronic conductivity likely by swelling the polymer and increasingthe distance between the conjugated chains, thus decreasing theelectronic transport. The dominating charge carrier is the electroniccharge carrier as also indicated by the small and constant value of theSeebeck coefficient versus RH.

c) PEDOT:PSS with PSSNa

In order to increase the ionic conductivity of PEDOT:PSS, thepolyanionic salt PSSNa is added in the PEDOT:PSSH emulsion, resulting inPEDOT:PSSH-PSSNa. Since PSSNa is not an electronic conductor, theelectrical conductivity in dry condition is mostly due to the electronictransport and it is 200 times lower than the pristine PEDOT:PSSH (FIG.25 a). As soon as the RH increases, the ionic conductivity dominates andcontinuously increases. Also the ionic Seebeck increases with thesimilar pace as in PEDOT:PSSH.

In conclusion, for conjugated polymer+polyelectrolyte materials, PFincreases at high humidity and follows the material sequence:PEDOT:PSSH-PEDOT:PSSH-PSSNa-PEDOT:PSSH-DEG. A maximum PF of 0.34μWm⁻²K⁻¹ can be gained for PEDOT:PSSH at 80% RH.

Results for Conjugated Polyelectrolytes

Derivatives of P3HT (a non-polyelectrolyte conjugated polymer), modifiedin the pendent group, have been introduced, where P3PT-COOH andP3PT-COOK bear the same anionic pendent but different counter-ions as H⁺and K⁺. TE properties for P3HT, P3PT-COOH and P3PT-COOK have beencompared. In dry condition, they exhibit similar conductivity and S,most probably due to electrons, as shown in Table 2. In hydrated films,as depicted in FIG. 3, electronic δ and S for P3HT and P3PT-COOH arealmost constant. For P3HT, it is reasonable since it is a pureelectronic conductor. Although P3PT-COOH possesses a carboxylic acidgroup, this group does not dissociate. The polymer is indeed insolublein water but may dissolve in dipolar aprotic solvents DMF, DMSO and NMP.Thus this polymer might be more appropriate when such solvent are used.This could be advantageous since they have a high boiling point andcannot evaporate as fast.

Therefore, proton can't contribute to the conductivity and S. As forwater soluble P3PT-COOK salt, K⁺ is fully dissociated and contributes tothe conductivity δ and S in the hydrated film. As observed, large ionicδ and S can be yielded in wet P3PT-COOK film. At 80% RH, δ can beenhanced about 4 orders of magnitudes while 18 times for S.Subsequently, an high PF up to 515.12 pWm⁻¹K⁻² can be achieved.

TABLE 1 Structures of non-conjugated polyelectrolyte and conjugatdpolymer-polyelectrolytes and their TE properties in dry films Dry Film SPF (mV/ (μW/ Charge δ (S/m) K) mK²) carrier Structure PSSNa 7.53 × 10⁻⁶≈0 ≈0 □₊

PEDOT:PSSH- DEG 5.30 0.007 0.026 e₊ PEDOT:PS SH mix with DEG PEDOT:PSSH17.1 0.012 0.003 e₊, □₊

PEDOT:PSSH- PSSNa 7.48 × 10⁻² 0.015 1.68 × 10⁻⁵ e₊, □₊ PEDOT:PS SH mixwith PSSNa

TABLE 2 Structures of conjugated polyelectrolytes and their TEproperties in dry films Dry Film S PF σ (S/m) (mV/K) (μW/mK²) Chargecarrier Structure P3HT 2.94 × 10⁻⁴ 0.568 9.49 × 10⁻⁴ e₊

P3PT-COOH 4.92 × 10⁻⁴ 0.581 1.66 × 10⁻⁴ e₊, □₊

P3PT-COOK 7.71 × 10⁻⁴ 0.472 1.72 × 10⁻⁴ e₊, □₊

Example 10 Crosslinked Polyelectrolyte

At humidity levels higher than 80% RH, the polyelectrolyte films canlose their mechanical integrity and dissolve in the absorbed water. Toprevent this a polyelectrolyte PSS:Na film was cross-linked using apolysiloxane crosslinker (Silquest-187A). This allows forcharacterization of the thermoelectric properties of the polyelectrolytein wet conditions. When a reservoir of 1M NaCl is connected to thecross-linked polyelectrolyte film, the polymer film swells to reachwater saturation. The ionic conductivity of this wet and salt-dopedpolyelectrolyte film reaches 1.2 S/m (FIG. 21 a) which is close to theionic conductivity of a liquid aqueous electrolyte. The crosslinked wetPSS:Na film possesses a large and positive ionic Seebeck coefficient of+47 mV/K (FIG. 21 a); thus leading to an very high power factor (σα²) of2680 μWm⁻¹K⁻² (FIG. 21 a).

The thermal conductivity of the wet crosslinked PSS:Na film soaked inNaCl solution reaches 0.49 Wm⁻¹K⁻¹ (FIG. 21 b). The cross-linked PSSfilm soaked with the salt solution reaches a very high ZT value of 1.6due to the advantageous ionic conductivity as compared to the filmexposed at 80% RH.

Example 11 The Thermoelectric Device as a Supercapacitor

In this example the conducting polymer poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) used as supercapacitorelectrodes. Conducting polymers transport both electronic and ioniccharge carriers such that the surface notion of the electric doublelayer capacitor becomes a bulk notion and the specific capacitancereaches 10⁵-10 ⁶ F/kg. The charge-discharge for this supercapacitor ismeasured by the output voltage vs. time as presented in FIG. 22 for oneheating-cooling cycle in N₂ atmosphere at 80% RH. When a temperaturegradient (ΔT=1.2 K) is applied, the thermo-electric effect acts like agenerator that is charging a PEDOT-PSS/PSS/PEDOT-PSS supercapacitor.When a stable temperature gradient is reached, the device is connectedto the a load resistance. The current generated is recorded for 40minutes and corresponds to an integrated charge of Q=3.55×10⁻⁵ coulombsstored in the PEDOT:PSS electrodes. Directly after the heatinghalf-cycle, the device is disconnected from the load resistor and thetemperature gradient gradually falls to zero. When ΔT=0 K, the opencircuit voltage of the charged supercapacitor is V_(oc)(ΔT=0)=55.3 mV,which is of the same order of magnitude but slightly lower than theSeebeck voltage (110 mV) measured at open circuit for ΔT=1.2 K (seeinset of FIG. 22). This indicates that the supercapacitor is not fullycharged in 40 min at this temperature gradient (as expected from thecharging curve that has not reached it minimum current as in FIG. 22 a).The specific capacitance is C_(s)=QV_(oc) ⁻¹M⁻¹=˜10⁵ Fkg⁻¹ (the mass ofthe PEDOT:PSS electrode is 0.32 mg). This value is in good agreementwith the measured specific capacitance by impedance spectroscopy(C_(s)=6×10⁴ F/kg, see FIG. 23) and corresponds to the values reportedin the literature for conducting polymers. The charged supercapacitor isthen connected to a small load resistor (50 kΩ) and its discharge ischaracterized. The output voltage decreases versus time. When thecurrent level drowns within the noise level, the two PEDOT:PSSelectrodes can be assumed to be back at the same oxidation level asbefore charging. The charge released during discharge is Q=3.47×10⁻⁵coulombs, thus smaller than the charge produced by the thermoelectriceffect. This is attributed to the self-discharge of the polarizedPEDOT:PSS electrodes. For the supercapacitor, the energy density is 173J/kg for ΔT=1.2K. The energy density increases quadratically with thetemperature gradient according to E=1/2C_(s)(ΔTα_(i))² (see FIG. 22 b).The extrapolated energy density for ΔT=10 K and ΔT=100 K are 12 kJ/kg(3.32 Wh/kg) and 1.2 MJ/kg (332 Wh/kg). Importantly, the energy densityfor larger temperature gradient can be high because the ionic Seebeckvoltage of the polyelectrolyte is exceptionally large. For the sake ofcomparison, Li-polymer batteries have an energy density of about 200Wh/kg.

Example 12 Measurement of Specific Capacitance by Impedance Spectroscopy

Devices were fabricated by sandwiching a 50 μm thick solid-state PSS:Nafilm between two PEDOT:PSS film electrodes, PEDOT:PSS/PSS:Na/PEDOT:PSS.When applying an alternating current (AC) voltage across thesupercapacitor, the polarization characteristics of the electrolytestrongly depends on the frequency. The device was is kept in a home-madeclimate chamber with saturated water atmosphere overnight. The devicewas connected to an impedance spectrometer through PEDOT:PSS electrodesand the frequency was swept from 10⁴ Hz to 10⁻² Hz. The device wasscanned with different AC varied from 1 V, 100 mV, 10 mV to 1 mV,respectively, which display similar capacitive behavior. The phase angleand capacitance versus frequency with AC at 100 mV are given in FIG. 23.The phase angle of the device reaches low angle (<−45°) at lowfrequency, implying that device showing a dominant capacitive behaviorat those frequencies. For the capacitance, it increases with thefrequency and saturates at low frequencies, around 7×10⁻⁴F. With themass of the PEDOT:PSS, the specific capacitance for the PEDOT:PSS wascalculated as about 6×10⁴ FKg⁻¹.

It should be noted that the invention has mainly been described abovewith reference to a few embodiments. However, as is readily appreciatedby a person skilled in the art, other embodiments than the onesdisclosed above are equally possible within the scope of the invention,as defined by the appended patent claims.

It is further noted that, in the claims, the word “comprising” does notexclude other elements or steps, and the indefinite article “a” or “an”does not exclude a plurality. A single apparatus or other unit mayfulfill the functions of several items recited in the claims. The merefact that certain features or method steps are recited in mutuallydifferent dependent claims does not indicate that a combination of thesefeatures or steps cannot be used to an advantage.

1. A thermoelectric device comprising a first electrode, a secondelectrode, and a conducting composition capable of conducting ions,wherein the first and second electrodes are ionically coupled via saidconducting composition such that an applied temperature difference oversaid conducting composition or an applied voltage over said electrodesfacilitate transport of ions to and/or from said electrodes via saidconducting composition, and wherein said conducting composition capableof conducting ions comprises a polymeric electrolyte.
 2. Athermoelectric device according to claim 1, wherein the conductingcomposition is further capable of conducting electrons.
 3. Athermoelectric device according to claim 1, wherein the conductingcomposition comprises at least one conducting polymer and at least onepolyelectrolyte, or at least one conjugated polyelectrolyte.
 4. Athermoelectric device according to claim 1, wherein the first and secondelectrodes comprise a material having a specific capacitance in therange of 10 F/g to 1000 F/g, for example a material having a specificsurface area in the range of 50 m²/g to 5000 m²/g or an electricallyconductive polymer composition capable of being reduced and/or oxidized.5. A thermoelectric device according to claim 1, further comprising atleast one ion reservoir at the junction between the conductingcomposition and said first and/or second electrodes.
 6. A thermoelectricdevice according to claim 1, wherein said first electrode, secondelectrode and/or conducting composition can be applied by liquiddeposition techniques.
 7. A thermoelectric device according claim 1,arranged on a flexible solid substrate.
 8. A thermoelectric deviceaccording to claim 1 for generating electric current comprising a firstleg connected to said first electrode and a second leg connected to saidsecond electrode, wherein said first and second legs are coupled via aconnector, wherein said first leg is connected to said first electrodeby being in ionic contact, said second leg is connected to said secondelectrode by being in ionic contact, and said connector is in ioniccontact with said first and said second legs; wherein said connectorcomprises a composition comprising mobile cations and mobile anions saiddevice further comprises a first ion reservoir being in ionic contactwith said first leg, and said first electrode and a second ion reservoirbeing in ionic contact with said second leg and said second electrode,wherein said first and second ion reservoirs and said connector arespatially isolated from each other; wherein said first leg comprises afirst conducting composition comprising a polymeric electrolyte capableof transporting cations from said connector to said first ion reservoir,said second leg comprises a second conducting composition comprising apolymeric electrolyte capable of transporting anions from said connectorto said second ion reservoir; and wherein said first electrode comprisesa layer of a first electrically conductive polymer composition capableof being reduced which is in ionic contact with said first ionreservoir, and said second electrode comprises a layer of a secondelectrically conductive polymer composition capable of being oxidizedwhich is in ionic contact with said second ion reservoir.
 9. Athermoelectric device according to claim 1 for generating a temperaturedifference comprising a first leg connected to said first electrode anda second leg connected to said second electrode, wherein said first andsecond legs are coupled via a connector, wherein said first leg isconnected to said first electrode by being in ionic contact, and saidsecond leg is connected to said second electrode by being in ioniccontact; said device further comprises a first ion reservoir comprisingmobile cations, being in ionic contact with said first leg, and saidfirst electrode and a second ion reservoir comprising mobile anions,being in ionic contact with said second leg and said second electrode,wherein said first and second ion reservoirs and said connector arespatially isolated from each other; wherein said first leg comprises afirst conducting composition comprising a polymeric electrolyte capableof transporting cations from said first ion reservoir to said connectorsaid second leg comprises a second conducting composition comprising apolymeric electrolyte capable of transporting anions from said secondion reservoir to said connector; wherein said connector comprises acation and anion transporting composition in ionic contact with saidfirst and said second legs; and wherein said first electrode comprises alayer of a first electrically conductive polymer composition capable ofbeing oxidized which is in direct contact with said first ion reservoir,and said second electrode comprises a layer of a second electricallyconductive polymer composition capable of being reduced which is indirect contact with said second ion reservoir.
 10. A method forgenerating electric current comprising the steps of: providing athermoelectric device according to claim 1, and applying a temperaturedifference over said conducting composition.
 11. A method according toclaim 10, comprising providing a thermoelectric device and providing afirst temperature in said connector and a second temperature in saidfirst and second ion reservoirs, wherein said first temperature is lowerthan said second temperature.
 12. A method for generating a temperaturedifference comprising the steps of: providing a thermoelectric deviceaccording to claim 1, and applying a potential difference between saidelectrodes.
 13. A method according to claim 12, comprising applying apotential difference between said first and second electrodes.
 14. Useof a thermoelectric device as defined in claim 1 as a temperaturesensor, or for charging a capacitor.
 15. Use of a polymeric compositioncapable of conducting both ions and electrons in a thermoelectricdevice.